Hume, A Treatise of Human Nature, Book 1, Part 2, Sect 5 "The Same Subject Continu'd" §§123-151

by Corry Shores
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David Hume

A Treatise of Human Nature

Book I: Of the Understanding

Part II: Of the Ideas of Space and Time

Section V: The Same Subject Continu'd

§123

The second part of Hume's system was: the idea of space or extension is nothing but the idea of visible or tangible points distributed in a certain order.

We cannot see voids, so we may form no ideas of vacuum or space containing nothing visible or tangible. (53d)

Hume will now address 3 objections to this principle. The consequence of one will provide a premise for the following.

§124

Philosophers have long debated whether or not space can be empty or not (vacuum vs plenum). But before we can refute or defend the existence of a vacuum, first we must properly conceive it. (54a.b)

§125

Hume will first show that the idea of a vacuum is possible.

Every thing that is conceivable is possible. Hence every idea that we have is possible.

We know the world now to be filled (that is, to be a plenum). Also, we may conceive there not being motion. Hence it is possible there is no motion.

Every idea that is distinguishable is also separable in the imagination. And every idea that is separable in the imagination is may be conceived as existing separately.

We can separately conceive neighboring particles of matter. Now, just because the window is square does not imply that everything else must be a square. We can conceive everything else separately from the window. Likewise, the existence of one particle of matter does not necessarily imply the existence of another particle of matter.

Thus, we may conceive that God could destroy one particle of matter without causing the others to move. So they could remain at rest.

Now consider a chamber filled with air. Hume now wonders what would happen if we removed all the air from the chamber, without touching the walls or causing them to move. We will consider some theories.

Some metaphysicians think that matter and extension are the same. So the annihilation of matter will also annihilate its extension. Thus in the same act, the distance between the walls would become nothing, and the walls then would touch.

Hume challenges these metaphysicians to conceive of how all six sides of the chamber will touch each other, without involving any motion or change of position.

For how can the two walls, that run from south to north, touch each other, while they touch the opposite ends of two walls, that run from east to west? And how can the floor and roof ever meet, while they are separated by the four walls, that lie in a contrary position? (55a)

But if we change their position, then we must also suppose they move. Yet, we said that no force acts on the walls so to cause them to move. However, if we conceive that there to be some thing between the walls, then we suppose that something new has been created between them. But, we began by supposing that there was an annihilation and not a creation. Thus we must consider a vacuum to exist. For otherwise we could not explain how the walls remain at rest while everything between them was deleted.

But keeping strictly to the two ideas of rest and annihilation, it is evident, that the idea, which results from them, is not that of a contact of parts, but something else; which is concluded to be the idea of a vacuum. (55b)

§126

Some others argue that the only way we can explain how one body moves from one place to another is if there is a vacuum for it to enter-into. Thus they claim that vacuums are necessary. Hume will let the natural philosophers decide this.

§127

To be certain that we understand this dispute, Hume will consider the nature and origin of the ideas involved.

We begin by noting that darkness is the lack of light. As a negation, darkness is not a positive idea. [See Bergson's Time and Free Will §36 where he claims that darkness is not somehow less than light, but rather qualitatively different.]

In the total darkness, someone with perfect vision has no more of a visual perception from shifting his eyes than a blind person would have. [Compare to Bergson's shooting star demonstration of the qualitative sensation of motion in §69] In fact, blind people would have no idea of what light and darkness are.

Thus if we remove visible objects from our vision, we cease having a visual impression. So we do not obtain an impression of matter-less extension. Hence "the idea of utter darkness can never be the same with that of vacuum." (56a)

§128

Hume now has us imagine that somehow the air is holding a man off the ground. And he moves from place-to-place by some "invisible power." His motion never varies. And he has no sense of orientation. Thus "he is sensible of nothing, and never receives the idea of extension, nor indeed any idea, from this invariable motion." (56b) However, he does have successions of impressions that give him a sense of time.

Nonetheless, he never has impressions that give him ideas of space or extension.

§129

So darkness that removes everything visible cannot give us the idea of a vacuum. And motion without tangible sensation of matter cannot give us the idea of extension. Now Hume asks, can these conditions convey these ideas when they are mixed with visible and tangible things?

§130

Some philosophers argue this: what we see appears to us as if it were a flat painting. And we only obtain a sense of depth ("different degrees of remoteness from ourselves") by use of our reason and not so much by means of our senses. [Now consider a child who sees the moon. Before knowing how far out-of-reach it is, he might try grasping it in his hands, or jumping with his arms stretched. Even a ladder will not get him nearer to it, despite the fact that the ladder "touches" the moon in his visual field the same way that it touches the roof.

So according to this theory, depth is something we come to deduce from other facts about the objects' accessibility. This also informs us about the nature of proportions and the way they change at different distances and angles. So in our vision, there is no visual information of what lies between us and a distant object. This lack of sensible matter then becomes considered as, but not perceived as, depth. In a similar manner, ] Hume has us hold our hands up to the blue sky above, spreading our fingers wide. We notice that our fingers a separated by "the blue colour of the firmament." Although the sky is so far away, it seems that any object could have been placed between our fingers. So, if no object were sensible between our fingers – all while our fingers were still visible – then we might obtain the idea of a vacuum.

In order, therefore, to know whether the sight can convey the impression and idea of a vacuum, we must suppose, that amidst an entire darkness, there are luminous bodies presented to us, whose light discovers only these bodies themselves, without giving us any impression of the surrounding objects.

So in this example, we see mostly darkness around us, except for certain illuminated objects. Regardless of their distance, they all at first could be aligned along the same vertical plane. We also think that if we saw something between them, we would think that there was matter in that space. But since we do not see anything between these objects, we believe that we obtain the impression of a vacuum between them.

§131

Consider parallel cases in the other senses. Darkness overwhelms us. And we see nothing. But we feel familiar objects around us that help us find our way. We touch one object, then reaching out for the next, we touch another object further away, and move that direction. So we have sensations, and intervals with no sensation. Hume wonders if these empty intervals also provide us with the idea of extension apart from extending bodies. (57a)

§132

Hume returns first to the case of the illuminated objects surrounded by utter darkness. Imagine that we see two objects, and the rest black. We can gauge how much distance is between them. And we would notice that distance increase or decrease if they move away or towards each other. But we know that we have no impression of the darkness. Yet we consider it as a distance. Hence perhaps by this means we might obtain the impression of a vacuum or pure extension that is "not only intelligible to the mind, but obvious to the very senses."(57bc)

§133

This is what we are naturally inclined to think. But Hume will show us otherwise. Consider we only see utter darkness. Then two illuminated objects appear, surrounded still by the dark. This changes nothing else. The rest of our field of vision is still just a negation of light. And the distance between them also is just darkness. It has no parts or composition. It is invariable and indivisible. This darkness gives us no more of a visual impression than what blind people have.

And as blindness and darkness afford us no ideas of extension, it is impossible that the dark and undistinguishable distance betwixt two bodies can ever produce that idea. (57d)

§134

When two objects appear in the absolute darkness, we obtain impressions of these uniquely illuminated objects. We do not also obtain a second category of impressions of the extension between them. [Imagine we are in a very thick forest. There is a full moon, but the canopy of leaves above us completely blocks its light. We wander in the darkness, and see two tree stumps in different parts of our vision. It seems that there are two small breaks in the canopy that let two separate beams of moonlight fall through the forest darkness. We can tell from their proportions how far away each one is. And given those distances from our position, we judge their extent of separation from the amount of motion our eyes make when moving between them.] The illuminated objects give us many different sorts of impressions. Some of them serve to indicate their distance from each other. We also have muscle sensations of our eyes' movement between the objects. All these factors combined allow us to determine the distance between the illuminated objects, even though we do not sense that distance by itself.

The angles, which the rays of light flowing from them, form with each other; the motion that is required in the eye, in its passage from one to the other; and the different parts of the organs, which are affected by them; these produce the only perceptions, from which we can judge of the distance. But as these perceptions are each of them simple and indivisible, they can never give us the idea of extension.

§135

Hume will further illustrate with the sense of touch, and the "imaginary distance or interval interposed betwixt tangible or solid objects." Consider two situations.

1) A man is suspended. He is supported by the air. He moves his arms and legs about. But he feels nothing.

2) A man makes a motion, then feels an object. He makes another motion, sensing nothing until encountering another object.

Hume wonders what is the difference in sensation between the man who touches nothing, and the man who touches nothing in-between instances of feeling objects. Hume says there is only one difference: the second man feels objects once in a while, but the first man never does. So when the second man feels nothing in-between objects, he has the same sensation as the first man who never feels anything. The man who feels nothing has no sense of extension while moving his arms. Likewise, the second man has no impression of extension in-between instances of contact.

And as that sensation is not capable of conveying to us an idea of extension, when unaccompanied with some other perception, it can no more give us that idea, when mixed with the impressions of tangible objects; since that mixture produces no alteration upon it.

§136

So motion and darkness alone do not give us an idea of a vacuum or extension without matter. Nor do they give us an impression when they are in-between other tangible or visual objects. We form these notions only by what is given to our senses, not by what is absent from our sensation. But recall in the case of the distant tree-stumps alit alone in the forest darkness. Certain impressions we did have of them indicated to us the distance between them. We did not sense the distance by looking at the interposing darkness. Yet, these other indicators are not so obvious to us as the darkness is. And if that darkness were not there, or if the empty motions in the air were filled with tactile sensation, then we would use what is given to us between the objects as indicating the extension separating them. So when we see darkness instead, we are inclined to attribute our sense of distance to the darkness rather than to the impressions we are actually having. (58c.d)

§137

Now consider the forest lit-up entirely. The two distant tree-stumps will reflect light to us at different angles whose relative measure depends on how far apart they are. And there will then be a continuum of other light-angle reflections from the things between them. Now, if the objects between them are no longer illuminated, still the light reflecting from the stumps will arrive to our eyes with a difference between their angles that is the same as when everything was lit.

Now imagine that there is a series of books held together between two marble book-ends. With our eyes closed, we start with our hands on one book-end, then feel all the variety of books between, then feel the marble book-end on the other side. Now imagine instead that we remove the books, but keep the book-ends equally apart. Again, we close our eyes, begin feeling the left book end, then we feel nothing between, and finally we feel the right book-end. In the first case, we judged the extent by how much we felt between. At the same time, we had a feeling of motion in our bodies. In both cases, we felt our bodies move through the same range of motion.

§138

Consider first the above example of the tree-stumps. When the things between them are not illuminated, we still may judge their distance by the angles of light reflecting from them. And also, if two other objects reflect light in the same way, we will also judge them to be just as distant as the tree stumps. So even though the extent between them is invisible, we see it in terms of what Bergson might call "representative sensations" that make it visible by means of other sensations that reflect the distance.

Now instead consider the example of the book-ends. Whether or not the books are between the book-ends, we judge the distance to be the same based on the fact that our bodies undergo the same range of motion in both cases. And if the objects were different than book-ends, so long as the range of motion is the same, so too will our distance-judgment remain the same. The feeling of motion is likewise something of a representative sensation that allows us still to feel the extent, only by a different tactile means.

Thus,

an invisible and intangible distance may be converted into a visible and tangible one, without any change on the distant objects.

§139

Consider also how such qualities as heat, cold, light, and attraction diminish as they become more distant from their sources. Hence even if we do not see the material between things or between parts of things, we can still detect such diminishments and judge distances based on them.

§140

So above we have seen different ways that the extensive distances may be conveyed to our senses without sensing all the material between the distant objects.

§141

Normally we are not aware of these other ways, so we mistakenly think we may sense pure extension itself, and not just extending things. The idea of an extension of empty space is very close to the idea of an extension of space we cannot directly sense. And it is a general fact of human nature that we confuse very similar ideas.

So we have this phenomenon of mistaking darkness and empty motion with pure extension or vacuum. We do it often, and Hume will discuss the causes. Soon we will see clearly the difference between the phenomena and their causes.

Normally when the causes for this mistake present themselves, we do not notice them. This makes it not so clear that we are committing the error. But after we see the causes distinctly from their effects, then we will know that even if we do not notice these causes, we can still expect that we are making the mistakes. In fact, Hume admits that his explanation of the causes might be wrong, but that will not make the mistakes any less present. If his explanation is false, that does not mean the phenomenon does not exist. Yet, it is natural for us to expect the explanation's truth or falsity to determine whether or not the phenomena exists. Hume will now explain this tendency.

§142

From the beginning, Hume has refrained from hypothesizing about the causes for certain properties of human understanding [see §8 where Hume explains that we should not go beyond experience in explaining causes, thus we will produce no hypotheses.] So he previously did not examine the causes for the relations of resemblance, contiguity, and causation [see §30 where he writes that there is a force of attraction in such associations, and its causes must remain mysterious.] Hume says he could have illustrated the cause by having us imagine a dissection of the brain. Now he will evoke this imagery to explain the mistakes arising from these three relations.

We envision that the mind is able to evoke any idea it wants. To do so, it

dispatches the spirits into that region of the brain, in which the idea is placed; these spirits always excite the idea, when they run precisely into the proper traces, and rummage that cell, which belongs to the idea. (60-61)

But,

their motion is seldom direct, and naturally turns a little to the one side or the other; for this reason the animal spirits, falling into the contiguous traces, present other related ideas in lieu of that, which the mind desired at first to survey. (61a)

However, we not often aware of these related ideas that are evoked.

This change we are not always sensible of; but continuing still the same train of thought, make use of the related idea, which is presented to us, and employ it in our reasoning, as if it were the same with what we demanded. (61b)

So because the contiguous idea is not directly relevant, its incorporation into our trains of thought cause many of the mistakes we see in philosophy.

§143

Consider the greatest source of errors: resemblances. To recall one idea, and to recall one resembling it, involves practically the same act of the mind. So not only are the ideas largely the same, so too are the mental acts that retrieve them.

wherever the actions of the mind in forming any two ideas are the same or resembling, we are very apt to confound these ideas, and take the one for the other. Of this we shall see many instances in the progress of this treatise. (61c)

Now consider a mistake of resemblance that is a bit more complex.

We see an extent of space. This impression is made-up of visible points placed in a certain order. From our sense of the quantity of those points, we might assess the distance between things we see. Thus we can convert these points into a measurement of distance. In this way, the visible points are a cause for the amount of distance we see. But that does not mean that the extension made-up by the points is the same idea as the idea for the distance between them. We have already seen that there is no empty extension. So causation is one reason we make this mistake. The other reason is that both the distance between things, and the extension of points between them, both have a similar effect on our senses. Hence resemblance concurrently leads to us mistaking distance for extension.

§144

Now that Hume has explained his principles, he may now address objections to his exposition, particularly ones from metaphysics and mechanics. Just because men dispute the concepts of vacuum and matter-less extension does not mean the ideas are real. Instead, it is more often the case that they mistake these concepts for closely related ideas.

§145

Recall Hume's example of the chamber whose air was annihilated. The walls did not move. So we continue having the same impressions of the walls. But the air is gone. We might then think that now we perceive the pure matter-less extension between the walls. But really, our visual impression of the air is no different than of the vacuum. So we obtained no additional ideas. Nor did our impression of filled-space simplify to an impression of empty-space.

§146

Now, consider we replace the air back into the empty chamber. This does not add any new extension. So it will not cause the walls to move further apart.

there is no obstacle to the conversion of the invisible and intangible distance into one that is visible and tangible. (63d)

Yet, we learn this first from experience.

§147

Still, some will object that Hume only accounts for the way objects affect the senses, and not the way they really are. Hume clarifies that we obtain our knowledge of these principles by experience alone, and he does not posit any hypotheses regarding their causes.

This invisible and intangible distance is also found by experience to contain a capacity of receiving body, or of becoming visible and tangible. Here is the whole of my system; and in no part of it have I endeavoured to explain the cause (63-64)

§148

I answer this objection, by pleading guilty, and by confessing that my intention never was to penetrate into the nature of bodies, or explain the secret causes of their operations. (64a)

Giving such theoretical accounts is not his aim, nor is it even possible for us to decide these issues.

such an enterprise is beyond the reach of human understanding, and that we can never pretend to know body otherwise than by those external properties, which discover themselves to the senses. (64ab)

Hume only wants to learn from experience how objects affect his senses, and the ways their impressions connect with each other.

§149

Hume notes two possible positions one may take on the issue of extension:

We consider for example the chamber whose air is annihilated. The empty space is an invisible and intangible distance. We would not sense it until something was apparently there. So we might more rightly call it the "capacity of becoming a visible and tangible distance."

Those who might call it a vacuum believe that you can have extension without matter. But then, by calling the space a vacuum, they have turned it into an entity of sorts. Only it is an intangible and invisible entity. They are not denying the vacuum, so they are not saying there is absolutely nothing between the walls and hence they collapse. No, rather, they are saying something interposes the walls, a vacuum, and so in one way they affirm that matter and extension are different, but they are also giving extension to a thing, even though it is not a sensible thing. So people who call that space a vacuum both affirm and deny that extension and matter are the same.

Or, one may not call that space a vacuum. That means they presume the walls will collapse. But that also means that the walls' motion happened without some force acting on them. It also means that everything else in the cosmos will shift towards that deleted space. And consider Spinoza's diagram from his Principles of Cartesian Philosophy.

When 8 takes 1's place, 1 moves into 2's place, and so on, until 7 takes 8's place. Normally, if the walls were to move inward, then the air would be forced out, which pushes a chain of objects into the place where the walls once were. But, if we deny the existence of vacuums, then the deletion of the air between the walls will cause motion that does not result in a circulation. For, there will not be any displacement of matter from inside the chamber. As well, normally for motion one object must penetrate another space. But according to those who deny vacuums, that extensive space disappears with the matter. So the walls will move, but they will not move into any space. So we have movement without change of place. Hume summarizes:

If you will not give it that name, motion is possible in a plenum, without any impulse in infinitum, without returning in a circle, and without penetration. (64c)

So either way, says Hume, there results a paradox. His point is that we cannot give a satisfactory account for what our experience tells us, which is merely that you can remove the air from a chamber. The theories explaining it are paradoxical. But at least Hume can explain what we actually experience. For, whether or not there is a vacuum, we know that we will not sense the extension until visible and tangible parts fill the space.

§150

Hume now addresses the question of whether or not we can conceive of time passing when nothing changes. Some note that you can only argue about ideas that you actually have. And we are arguing about time passing without objects changing. So to argue about it, we must actually have a conception of what it is. [And, as Hume often notes, whatever we can conceive is thereby possible.]

But Hume asks them to tell us how we obtained that idea.

Does it arise from an impression of sensation or of reflection? Point it out distinctly to us, that we may know its nature and qualities. (65a)

However, no one can point-out any sort of impression that might allow us to clearly conceive of time without change. So Hume challenges the premise that we are actually arguing about some idea we have in mind. For, we have no such idea, and we have never experienced any such phenomena.

§151

So we cannot show any instance where we obtain this impression directly. But, we still believe we have the idea of time without change. Hume will now describe the sorts of experiences that give us this mistaken impression.

We notice that there is a continual succession of perceptions in our mind. Thus the idea of time is "ever present with us."

Imagine that we see a statue at five 'o clock. Then we see it again at six. In the mean-time, we had a succession of other impressions that gave us a sense of time. Then, we interpose that sense of duration between the two appearances of the statue. By doing so, we are pretending that it was under the influence of a series of changes corresponding to our succession of impressions. But what we see is that the object did not change.

So, we obtain our sense of time from the succession of impressions. Two impressions in the series were of the statue. Both those impressions were the same. In between those impressions was a succession that gave us a sense of duration. Then, we attributed that duration as a property to the idea of the unchanging statue. But we never sensed the 1 hour duration during which the statue remained unchanged. To do so, we would need to see the statue. But then, we would not sense the time, because there would no longer be a succession of impressions. For, the statue does not change. It's only when we attend to changing things that we may sense time again. So we confuse

1) the durations arising from a succession of impressions. This succession comes between two impressions of an unchanging object. This sense of time we mistake for

2) the unsensed interval between the two impressions of the unchanging object.

In this way we erroneously think we have the idea of a pure time passing without changing objects.

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From the text:

Sect. v. the Same Subject Continued.

If the second part of my system be true, that the idea of space or extension is nothing but the idea of visible or tangible points distributed in a certain order; it follows, that we can form no idea of a vacuum, or space, where there is nothing visible or tangible. This gives rise to three objections, which I shall examine together, because the answer I shall give to one is a consequence of that which I shall make use of for the others.

First, It may be said, that men have disputed for many ages concerning a vacuum and a plenum, without being able to bring the affair to a final decision; and philosophers, even at this day, think themselves at liberty to take part on either side, as their fancy leads them. But whatever foundation there may be for a controversy concerning the things themselves, it may be pretended, that the very dispute is decisive concerning the idea, and that it is impossible men coued so long reason about a vacuum, and either refute or defend it, without having a notion of what they refuted or defended.

Secondly, If this argument should be contested, the reality or at least the possibility of the idea of a vacuum may be proved by the following reasoning. Every idea is possible, which is a necessary and infallible consequence of such as are possible. Now though we allow the world to be at present a plenum, we may easily conceive it to be deprived of motion; and this idea will certainly be allowed possible. It must also be allowed possible, to conceive the annihilation of any part of matter by the omnipotence of the deity, while the other parts remain at rest. For as every idea, that is distinguishable, is separable by the imagination; and as every idea, that is separable by the imagination, may be conceived to be separately existent; it is evident, that the existence of one particle of matter, no more implies the existence of another, than a square figure in one body implies a square figure in every one. This being granted, I now demand what results from the concurrence of these two possible ideas of rest and annihilation, and what must we conceive to follow upon the annihilation of all the air and subtile matter in the chamber, supposing the walls to remain the same, without any motion or alteration? There are some metaphysicians, who answer, that since matter and extension are the same, the annihilation of one necessarily implies that of the other; and there being now no distance betwixt the walls of the chamber, they touch each other; in the same manner as my hand touches the paper, which is immediately before me. But though this answer be very common, I defy these metaphysicians to conceive the matter according to their hypothesis, or imagine the floor and roof, with all the opposite sides of the chamber, to touch each other, while they continue in rest, and preserve the same position. For how can the two walls, that run from south to north, touch each other, while they touch the opposite ends of two walls, that run from east to west? And how can the floor and roof ever meet, while they are separated by the four walls, that lie in a contrary position? If you change their position, you suppose a motion. If you conceive any thing betwixt them, you suppose a new creation. But keeping strictly to the two ideas of rest and annihilation, it is evident, that the idea, which results from them, is not that of a contact of parts, but something else; which is concluded to be the idea of a vacuum.

The third objection carries the matter still farther, and not only asserts, that the idea of a vacuum is real and possible, but also necessary and unavoidable. This assertion is founded on the motion we observe in bodies, which, it is maintained, would be impossible and inconceivable without a vacuum, into which one body must move in order to make way for another. I shall not enlarge upon this objection, because it principally belongs to natural philosophy, which lies without our present sphere.

In order to answer these objections, we must take the matter pretty deep, and consider the nature and origin of several ideas, lest we dispute without understanding perfectly the subject of the controversy. It is evident the idea of darkness is no positive idea, but merely the negation of .light, or more properly speaking, of coloured and visible objects. A man, who enjoys his sight, receives no other perception from turning his eyes on every side, when entirely deprived of light, than what is common to him with one born blind; and it is certain such-a-one has no idea either of light or darkness. The consequence of this is, that it is not from the mere removal of visible objects we receive the impression of extension without matter; and that the idea of utter darkness can never be the same with that of vacuum.

Suppose again a man to be Supported in the air, and to be softly conveyed along by some invisible power; it is evident he is sensible of nothing, and never receives the idea of extension, nor indeed any idea, from this invariable motion. Even supposing he moves his limbs to and fro, this cannot convey to him that idea. He feels in that case a certain sensation or impression, the parts of which are successive to each other, and may give him the idea of time: But certainly are not disposed in such a manner, as is necessary to convey the idea of s ace or the idea of space or extension.

Since then it appears, that darkness and motion, with the utter removal of every thing visible and tangible, can never give us the idea of extension without matter, or of a vacuum; the next question is, whether they can convey this idea, when mixed with something visible and tangible?

It is commonly allowed by philosophers, that all bodies, which discover themselves to the eye, appear as if painted on a plain surface, and that their different degrees of remoteness from ourselves are discovered more by reason than by the senses. When I hold up my hand before me, and spread my fingers, they are separated as perfectly by the blue colour of the firmament, as they coued be by any visible object, which I coued place betwixt them. In order, therefore, to know whether the sight can convey the impression and idea of a vacuum, we must suppose, that amidst an entire darkness, there are luminous bodies presented to us, whose light discovers only these bodies themselves, without giving us any impression of the surrounding objects.

We must form a parallel supposition concerning the objects of our feeling. It is not proper to suppose a perfect removal of all tangible objects: we must allow something to be perceived by the feeling; and after an interval and motion of the hand or other organ of sensation, another object of the touch to be met with; and upon leaving that, another; and so on, as often as we please. The question is, whether these intervals do not afford us the idea of extension without body?

To begin with the first case; it is evident, that when only two luminous bodies appear to the eye, we can perceive, whether they be conjoined or separate: whether they be separated by a great or small distance; and if this distance varies, we can perceive its increase or diminution, with the motion of the bodies. But as the distance is not in this case any thing coloured or visible, it may be thought that there is here a vacuum or pure extension, not only intelligible to the mind, but obvious to the very senses.

This is our natural and most familiar way of thinking; but which we shall learn to correct by a little reflection. We may observe, that when two bodies present themselves, where there was formerly an entire darkness, the only change, that is discoverable, is in the appearance of these two objects, and that all the rest continues to be as before, a perfect negation of light, and of every coloured or visible object. This is not only true of what may be said to be remote from these bodies, but also of the very distance; which is interposed betwixt them; that being nothing but darkness, or the negation of light; without parts, without composition, invariable and indivisible. Now since this distance causes no perception different from what a blind man receives from his eyes, or what is conveyed to us in the darkest night, it must partake of the same properties: And as blindness and darkness afford us no ideas of extension, it is impossible that the dark and undistinguishable distance betwixt two bodies can ever produce that idea.

The sole difference betwixt an absolute darkness and the appearance of two or more visible luminous objects consists, as I said, in the objects themselves, and in the manner they affect our senses. The angles, which the rays of light flowing from them, form with each other; the motion that is required in the eye, in its passage from one to the other; and the different parts of the organs, which are affected by them; these produce the only perceptions, from which we can judge of the distance. But as these perceptions are each of them simple and indivisible, they can never give us the idea of extension.

We may illustrate this by considering the sense of feeling, and the imaginary distance or interval interposed betwixt tangible or solid objects. I suppose two cases, viz. that of a man supported in the air, and moving his limbs to and fro, without meeting any thing tangible; and that of a man, who feeling something tangible, leaves it, and after a motion, of which he is sensible, perceives another tangible object; and I then ask, wherein consists the difference betwixt these two cases? No one will make any scruple to affirm, that it consists meerly in the perceiving those objects, and that the sensation, which arises from the motion, is in both cases the same: And as that sensation is not capable of conveying to us an idea of extension, when unaccompanyed with some other perception, it can no more give us that idea, when mixed with the impressions of tangible objects; since that mixture produces no alteration upon it.

But though motion and darkness, either alone, or attended with tangible and visible objects, convey no idea of a vacuum or extension without matter, yet they are the causes why we falsly imagine we can form such an idea. For there is a close relation betwixt that motion and darkness, and a real extension, or composition of visible and tangible objects.

First, We may observe, that two visible objects appearing in the midst of utter darkness, affect the senses in the same manner, and form the same angle by the rays, which flow from them, and meet in the eye, as if the distance betwixt them were find with visible objects, that give us a true idea of extension. The sensation of motion is likewise the same, when there is nothing tangible interposed betwixt two bodies, as when we feel a compounded body, whose different parts are placed beyond each other.

Secondly, We find by experience, that two bodies, which are so placed as to affect the senses in the same manner with two others, that have a certain extent of visible objects interposed betwixt them, are capable of receiving the same extent, without any sensible impulse or penetration, and without any change on that angle, under which they appear to the senses. In like manner, where there is one object, which we cannot feel after another without an interval, and the perceiving of that sensation we call motion in our hand or organ of sensation; experience shews us, that it is possible the same object may be felt with the same sensation of motion, along with the interposed impression of solid and tangible objects, attending the sensation. That is, in other words, an invisible and intangible distance may be converted into a visible and tangible one, without any change on the distant objects.

Thirdly, We may observe, as another relation betwixt these two kinds of distance, that they have nearly the same effects on every natural phaenomenon. For as all qualities, such as heat, cold, light, attraction, &c. diminish in proportion to the distance; there is but little difference observed, whether this distance be marled out by compounded and sensible objects, or be known only by the manner, in which the distant objects affect the senses.

Here then are three relations betwixt that distance, which conveys the idea of extension, and that other, which is not filled with any coloured or solid object. The distant objects affect the senses in the same manner, whether separated by the one distance or the other; the second species of distance is found capable of receiving the first; and they both equally diminish the force of every quality.

These relations betwixt the two kinds of distance will afford us an easy reason, why the one has so often been taken for the other, and why we imagine we have an idea of extension without the idea of any object either of the sight or feeling. For we may establish it as a general maxim in this science of human nature, that wherever there is a close relation betwixt two ideas, the mind is very apt to mistake them, and in all its discourses and reasonings to use the one for the other. This phaenomenon occurs on so many occasions, and is of such consequence, that I cannot forbear stopping a moment to examine its causes. I shall only premise, that we must distinguish exactly betwixt the phaenomenon itself, and the causes, which I shall assign for it; and must not imagine from any uncertainty in the latter, that the former is also uncertain. The phaenomenon may be real, though my explication be chimerical. The falshood of the one is no consequence of that of the other; though at the same time we may observe, that it is very natural for us to draw such a consequence; which is an evident instance of that very principle, which I endeavour to explain.

When I received the relations of resemblance, contiguity and causation, as principles of union among ideas, without examining into their causes, it was more in prosecution of my first maxim, that we must in the end rest contented with experience, than for want of something specious and plausible, which I might have displayed on that subject. It would have been easy to have made an imaginary dissection of the brain, and have shewn, why upon our conception of any idea, the animal spirits run into all the contiguous traces, and rouze up the other ideas, that are related to it. But though I have neglected any advantage, which I might have drawn from this topic in explaining the relations of ideas, I am afraid I must here have recourse to it, in order to account for the mistakes that arise from these relations. I shall therefore observe, that as the mind is endowed with a power of exciting any idea it pleases; whenever it dispatches the spirits into that region of the brain, in which the idea is placed; these spirits always excite the idea, when they run precisely into the proper traces, and rummage that cell, which belongs to the idea. But as their motion is seldom direct, and naturally turns a little to the one side or the other; for this reason the animal spirits, falling into the contiguous traces, present other related ideas in lieu of that, which the mind desired at first to survey. This change we are not always sensible of; but continuing still the same train of thought, make use of the related idea, which is presented to us, and employ it in our reasoning, as if it were the same with what we demanded. This is the cause of many mistakes and sophisms in philosophy; as will naturally be imagined, and as it would be easy to show, if there was occasion.

Of the three relations above-mentioned that of resemblance is the most fertile source of error; and indeed there are few mistakes in reasoning, which do not borrow largely from that origin. Resembling ideas are not only related together, but the actions of the mind, which we employ in considering them, are so little different, that we are not able to distinguish them. This last circumstance is of great consequence, and we may in general observe, that wherever the actions of the mind in forming any two ideas are the same or resembling, we are very apt to confound these ideas, and take the one for the other. Of this we shall see many instances in the progress of this treatise. But though resemblance be the relation, which most readily produces a mistake in ideas, yet the others of causation and contiguity may also concur in the same influence. We might produce the figures of poets and orators, as sufficient proofs of this, were it as usual, as it is reasonable, in metaphysical subjects to draw our arguments from that quarter. But lest metaphysicians should esteem this below their dignity, I shall borrow a proof from an observation, which may be made on most of their own discourses, viz. that it is usual for men to use words for ideas, and to talk instead of thinking in their reasonings. We use words for ideas, because they are commonly so closely connected that the mind easily mistakes them. And this likewise is the reason, why we substitute the idea of a distance, which is not considered either as visible or tangible, in the room of extension, which is nothing but a composition of visible or tangible points disposed in a certain order. In causing this mistake there concur both the relations of causation and resemblance. As the first species of distance is found to be convertible into the second, it is in this respect a kind of cause; and the similarity of their manner of affecting the senses, and diminishing every quality, forms the relation of resemblance.

After this chain of reasoning and explication of my principles, I am now prepared to answer all the objections that have been offered, whether derived from metaphysics or mechanics. The frequent disputes concerning a vacuum, or extension without matter prove not the reality of the idea, upon which the dispute turns; there being nothing more common, than to see men deceive themselves in this particular; especially when by means of any close relation, there is another idea presented, which may be the occasion of their mistake.

We may make almost the same answer to the second objection, derived from the conjunction of the ideas of rest and annihilation. When every thing is annihilated in the chamber, and the walls continue immoveable, the chamber must be conceived much in the same manner as at present, when the air that fills it, is not an object of the senses. This annihilation leaves to the eye, that fictitious distance, which is discovered by the different parts of the organ, that are affected, and by the degrees of light and shade;—and to the feeling, that which consists in a sensation of motion in the hand, or other member of the body. In vain should we search any farther. On whichever side we turn this subject, we shall find that these are the only impressions such an object can produce after the supposed annihilation; and it has already been remarked, that impressions can give rise to no ideas, but to such as resemble them.

Since a body interposed betwixt two others may be supposed to be annihilated, without producing any change upon such as lie on each hand of it, it is easily conceived, how it may be created anew, and yet produce as little alteration. Now the motion of a body has much the same effect as its creation. The distant bodies are no more affected in the one case, than in the other. This suffices to satisfy the imagination, and proves there is no repugnance in such a motion. Afterwards experience comes in play to persuade us that two bodies, situated in the manner above-described, have really such a capacity of receiving body betwixt them, and that there is no obstacle to the conversion of the invisible and intangible distance into one that is visible and tangible. However natural that conversion may seem, we cannot be sure it is practicable, before we have had experience of it.

Thus I seem to have answered the three objections above-mentioned; though at the same time I am sensible, that few will be satisfyed with these answers, but will immediately propose new objections and difficulties. It will probably be said, that my reasoning makes nothing to the matter in hands and that I explain only the manner in which objects affect the senses, without endeavouring to account for their real nature and operations. Though there be nothing visible or tangible interposed betwixt two bodies, yet we find BY EXPERIENCE, that the bodies may be placed in the same manner, with regard to the eye, and require the same motion of the hand in passing from one to the other, as if divided by something visible and tangible. This invisible and intangible distance is also found by experience to contain a capacity of receiving body, or of becoming visible and tangible. Here is the whole of my system; and in no part of it have I endeavoured to explain the cause, which separates bodies after this manner, and gives them a capacity of receiving others betwixt them, without any impulse or penetration.

I answer this objection, by pleading guilty, and by confessing that my intention never was to penetrate into the nature of bodies, or explain the secret causes of their operations. For besides that this belongs not to my present purpose, I am afraid, that such an enterprise is beyond the reach of human understanding, and that we can never pretend to know body otherwise than by those external properties, which discover themselves to the senses. As to those who attempt any thing farther, I cannot approve of their ambition, till I see, in some one instance at least, that they have met with success. But at present I content myself with knowing perfectly the manner in which objects affect my senses, and their connections with each other, as far as experience informs me of them. This suffices for the conduct of life; and this also suffices for my philosophy, which pretends only to explain the nature and causes of our perceptions, or impressions and ideas⁴.

⁴ As long as we confine our speculations to the appearances of objects to our senses, without entering into disquisitions concerning their real nature and operations, we are safe from all difficulties, and can never be embarrassed by any question. Thus, if it be asked, if the invisible and intangible distance, interposed betwixt two objects, be something or nothing: It is easy to answer, that it is SOMETHING, VIZ. a property of the objects, which affect the SENSES after such a particular manner. If it be asked whether two objects, having such a distance betwixt them, touch or not: it may be answered, that this depends upon the definition of the word, TOUCH. If objects be said to touch, when there is nothing SENSIBLE interposed betwixt them, these objects touch: it objects be said to touch, when their IMAGES strike contiguous parts of the eye, and when the hand FEELS both objects successively, without any interposed motion, these objects do not touch. The appearances of objects to our senses are all consistent; and no difficulties can ever arise, but from the obscurity of the terms we make use of.

If we carry our enquiry beyond the appearances of objects to the senses, I am afraid, that most of our conclusions will be full of scepticism and uncertainty. Thus if it be asked, whether or not the invisible and intangible distance be always full of body, or of something that by an improvement of our organs might become visible or tangible, I must acknowledge, that I find no very decisive arguments on either side; though I am inclined to the contrary opinion, as being more suitable to vulgar and popular notions. If THE NEWTONIAN philosophy be rightly understood, it will be found to mean no more. A vacuum is asserted: That is, bodies are said to be placed after such a manner, is to receive bodies betwixt them, without impulsion or penetration. The real nature of this position of bodies is unknown. We are only acquainted with its effects on the senses, and its power of receiving body. Nothing is more suitable to that philosophy, than a modest scepticism to a certain degree, and a fair confession of ignorance in subjects, that exceed all human capacity.

I shall conclude this subject of extension with a paradox, which will easily be explained from the foregoing reasoning. This paradox is, that if you are pleased to give to the in-visible and intangible distance, or in other words, to the capacity of becoming a visible and tangible distance, the name of a vacuum, extension and matter are the same, and yet there is a vacuum. If you will not give it that name, motion is possible in a plenum, without any impulse in infinitum, without returning in a circle, and without penetration. But however we may express ourselves, we must always confess, that we have no idea of any real extension without filling it with sensible objects, and conceiving its parts as visible or tangible.

As to the doctrine, that time is nothing but the manner, in which some real objects exist; we may observe, that it is liable to the same objections as the similar doctrine with regard to extension. If it be a sufficient proof, that we have the idea of a vacuum, because we dispute and reason concerning it; we must for the same reason have the idea of time without any changeable existence; since there is no subject of dispute more frequent and common. But that we really have no such idea, is certain. For whence should it be derived? Does it arise from an impression of sensation or of reflection? Point it out distinctly to us, that we may know its nature and qualities. But if you cannot point out any such impression, you may be certain you are mistaken, when you imagine you have any such idea.

But though it be impossible to shew the impression, from which the idea of time without a changeable existence is derived; yet we can easily point out those appearances, which make us fancy we have that idea. For we may observe, that there is a continual succession of perceptions in our mind; so that the idea of time being for ever present with us; when we consider a stedfast object at five-a-clock, and regard the same at six; we are apt to apply to it that idea in the same manner as if every moment were distinguished by a different position, or an alteration of the object. The first and second appearances of the object, being compared with the succession of our perceptions, seem equally removed as if the object had really changed. To which we may add, what experience shews us, that the object was susceptible of such a number of changes betwixt these appearances; as also that the unchangeable or rather fictitious duration has the same effect upon every quality, by encreasing or diminishing it, as that succession, which is obvious to the senses. From these three relations we are apt to confound our ideas, and imagine we can form the idea of a time and duration, without any change or succession.

From:

Hume, David. A Treatise of Human Nature. Ed. L.A Selby-Bigge. Oxford: Clarendon Press, 1979.

Text available online at:

http://ebooks.adelaide.edu.au/h/hume/david/h92t/

PDF available at:

http://ia341013.us.archive.org/3/items/treatiseofhumann01humeuoft/treatiseofhumann01humeuoft.pdf

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http://flickr.com/photos/markdodds/431359594/sizes/o/

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