Pirates & Revolutionaries: Hume, A Treatise of Human Nature, Book 1, Part 3, Sect 6 Of the Inference From the Impression to the Idea. §§204-219

by Corry Shores
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[The following is summary, up to the end where I reproduce this section in full. My commentary is in brackets. Paragraph headings are my own.]

David Hume

A Treatise of Human Nature

Book I: Of the Understanding

Part III: Of Knowledge and Probability

Section VI: Of the Inference From the Impression to the Idea.

§204 Causes and Effects do not by themselves Imply Each Other

Previously we noted how we make inferences from causes to effects. We saw that the causally related objects do not exhibit qualities that would necessarily relate them as causes to effects.

There is no object, which implies the existence of any other if we consider these objects in themselves, and never look beyond the ideas which we form of them. (86-87)

If we were to draw an inference, we would obtain knowledge.

Now, consider when we know something true like "if it is cloudy, the sun will not shine."

If we thought instead that "if it is cloudy, the sun will shine down upon us," than we would be thinking falsely. So if something is true, it cannot be otherwise. For if a statement said something different than the truth, then it would be a falsification of what is true. However, a causal inference

would amount to knowledge, and would imply the absolute contradiction and impossibility of conceiving any thing different. (87a)

But, if a cause and an effect are distinct ideas, then we may conceive them separately. So that means they do not necessarily need to be together. We may conceive of cloudy skies. And we may conceive of skies that are not lit by the sun. Each of these ideas stands fine on its own. So there is no necessity for us to always infer a lack of sun from clouds. For the sky also clouds-over at night. And, when thinking of overcast skies, we could have also inferred that we will be melancholy under the overbearing clouds. Again we see that there is no necessary connection between cloudiness and lack of sunshine.

When we pass from a present impression to the idea of any object, we might possibly have separated the idea from the impression, and have substituted any other idea in its room. (87b)

§205 Our Unceremonious Ritual of Linking Causes and Effects

Let's imagine instead that somehow the causal relations between causes and effects were implied in their essences. Here we would know that overcast skies prevent the sun from shining even if we never saw clouds or darkness ever in our lives. But we discovered that these impressions and ideas do not imply each other. So we cannot know this causal relation prior to our experiences of clouds and dim skies. In other words, we cannot have a priori knowledge of causal relations. The only other option is that we learn them after (posteriorly) to experience. This would be a posteriori knowledge.

It is therefore by EXPERIENCE only, that we can infer the existence of one object from that of another. (87b)

Hume proceeds to explain the nature of experiences that lead to causal associations. We begin with his example.

a) We remember numerous impressions of a type of object we call "flame."

b) We also remember that consistently whenever we had the impression of a flame, we also had a type of sensation we call "heat."

c) We do not only recall each idea by themselves. We also call to our mind their constant conjunction in all past occurrences.

d) "Without any farther ceremony, we call the one cause and the other effect, and infer the existence of the one from that of the other. " (87c)

Hence more generally, our experiences lead to causal associations when:

1) we remember frequent instances [or repetitions] of one species of objects, and

2) we also remember that objects of another species consistently attended them. Both types of objects occur together in "a regular order of contiguity and succession." (87b)

We will now distinguishing our mental activities when

a) we first learn of the causal relations between objects, and when

b) we reason on the basis of these relations.

1) When we first learn the conjunction of particular causes and effects, both the causes and the effects have already been perceived by the senses, and they are remembered.

However,

2) When we infer effects from causes, we only perceive or remember one of the objects, and then our minds supply the other object in the way we experienced it as occurring contiguously. [Specifically Hume writes: "In all those instances, from which we learn the conjunction of particular causes and effects, both the causes and effects have been perceiv'd by the senses, and are remember'd But in all cases, wherein we reason concerning them, there is only one perceiv'd or remember'd, and the other is supplyed in conformity to our past experience." p87c]

§206 Do We Conclude that Causes and Effects are Constantly Conjoined because They are Necessarily Connected? Or Do We Conclude that Causes and Effects are Necessarily Connected because They are Constantly Conjoined?

We previously noted that the only two relations we find between causes and effects are contiguity and succession [See sections §§169-176]. But in our further analysis we have uncovered another relation: constant conjunction.

Contiguity and succession are not sufficient to make us pronounce any two objects to be cause and effect, unless we perceive, that these two relations are preserv'd in several instances. (87d)

But there is now a new problem we must grasp. Causally related objects are found to be placed contiguously and successively. And it is on account of their repetitions or constant conjunction that they become causally related.

Now, we make inferences from causes. These inferences are possible because causes and effects are considered to be connected necessarily. And, we experience causes as being constantly conjoined with their effects. So there are two possibilities regarding the constant conjunctions and necessarily connections:

1) The constant conjunction of causes and effect (their repetition) associates ideas we already have in our mind. And, they are associated in a way that does not add a new idea. For, the idea of "constant conjunction" will be the same each time. Thus some argue that

As our senses shew us in one instance two bodies, or motions, or qualities in certain relations of success and contiguity; so our memory presents us only with a multitude of instances, wherein we always find like bodies, motions, or qualities in like relations. From the mere repetition of any past impression, even to infinity, there never will arise any new original idea, such as that of a necessary connexion; and the number of impressions has in this case no more effect than if we confin'd ourselves to one only. (88b emphasis mine)

So in this case, it would seem that the objects are already necessarily connected. Our minds would then make the inference from cause to effect because of this necessary connection. However, the other possibility is that

2) the necessity of the connection between cause and effect is a result of our making inferences from cause to effect. Because the necessary connection results from the inference, we obtain a new idea that was not inherently there between the impressions. For, just because they repeatedly occur together does not thereby imply they must necessarily always forever occur together.

Hume says that our analysis so far suggests the first possibility, where it seems we do not add new ideas when infering effects from causes. However, Hume will continue this line of thought to see if perhaps the second possibility is not really the case.

Perhaps 'twill appear in the end, that the necessary connexion depends on the inference, instead of the inference’s depending on the necessary connexion. (88c)

§207 Can We Understand or Imagine Causal Connections?

So, we have past experiences of objects. We find that they are constantly conjoined.

[Deleuze describes it as a repetition of AB, AB, AB, A... like the 'tic-tock' of a clock.]

Later, our mind makes a transition. It begins from an impression present to the memory or senses, which it considers the cause. Then it moves to the idea of an object, which it considers the effect.

Hume's next question is:

a) does the understanding produce the idea, or does the imagination. In other words,

b) does reason determine the transition, or does an association and relation of perceptions determine the transition.

[This might be the difference between an active or a passive synthesis.]

Consider if it is the understanding or reason that makes the transition. This means we are dealing with concepts for things. Perhaps everytime we eat a certain spicy Indian dish, we get indigestion. So we have a concept of this Indian dish and a concept of indigestion. We find a necessary connection between these concepts, beecause every time we experience them they are constantly conjoined.

Now, if the objects differed slightly each time we experienced these conjunctions, then they would not fit under the concepts that we causally connect. So, for us to make the causal transition between objects, each instance must resemble the rest. And hence each time we have new experiences of them, they must also resemble the past instances.

Also, if the way things happen in nature were to change, then so too would the causal relations between certain things. But if we infer effects from causes by means of our reason, then we would be making wrong inferences once nature's causal relations change. Thus

If reason determin'd us, it would proceed upon that principle, that instances, of which we have had no experience, must resemble those, of which we have had experience, and that the course of nature continues always uniformly the same. (89a)

[If however our imaginations make the transition by passive associations of perceptions, then we can experience variations. For there would not be any rigid concepts that our experiences must adhere-to. Rather there would be changing habits of causally associating objects based on the variations of their appearances.]

In order to know for sure if it is by reason that we make causal connections, Hume will examine all the arguments in favor of this position. Our reason would infer effects from causes on the basis of either knowledge or probability. So Hume will now examine degrees of evidence to see if they provide for reason to make the causal transition.

§208 If Nature can Change Course, then Alien Experiences are Possible

Hume's method of reasoning considers all imaginable things to be possible [for more see §73.]

To form a clear idea of any thing, is an undeniable argument for its possibility, and is alone a refutation of any pretended demonstration against it. (89c)

And, we can imagine that the principles of nature might change [if but slightly. See Deleuze's Difference and Repetition §4 for his discussion of this possibility.] In other words, we can conceive of a change in the "course of nature," so it must be a possibility. But, those who argue that new experiences must always resemble old ones are also thereby arguing that it is impossible to experience a difference on account of nature's course changing a bit. Yet we see that instead it is imaginable and thus possible. Thus,

Our foregoing method of reasoning will easily convince us, that there can be no demonstrative arguments to prove, that those instances, of which we have, had no experience, resemble those, of which we have had experience. (89b)

§209 Inducing the Unknown

We think that it is highly probable that the sun will rise tomorrow. And yet many of us are not astrophysicists. We draw this conclusion because we have had sense-impressions of the constant conjunction between the sun rising and the repetition of new days. But we also are drawing an inductive inference. So that means we are also dealing with concepts as well, and not just impressions. So probability-based causal inferences are founded both on impressions of our memory and senses, as well as on our ideas.

When for example we infer that the sun will rise tomorrow, we are connecting past impressions of the sun rising with the idea of the next sunup, which we have neither seen nor remembered.

'Tis therefore necessary, that in all probable reasonings there be something present to the mind, either seen or remember'd; and that from this we infer something connected with it, which is not seen nor remember'd.

§210 Probably Causal Relations are not Based on Probability

We previously examined all the types of connections between ideas. We see that of them, only cause and effect take us beyond our immediate sense and memory impressions. So when we see something we take to be a cause, we presume that the effect is also present, even though we might not also be perceiving it.

The idea of cause and effect is deriv'd from experience, which informs us, that such particular objects, in all past instances, have been constantly conjoin'd with each other: And as an object similar to one of these is suppos'd to be immediately present in its impression, we thence presume on the existence of one similar to its usual attendant. (89-90)

[Now consider when we infer that the sun will arise tomorrow. For, we notice that always before dawn, we experienced a similar morning darkess. And always after dawn we experienced daylight. So the next time we experience this similar morning darkness, we infer that the sun will arise that day, even though we have not yet experienced it. In the past we have always experienced the two objects being conjoined. So we think there is a necessary connection between them. This is why tomorrow morning before dawn, we will conclude that the sun will shortly arise. But we have not yet seen it come-up. So it is not necessary in this instance. It is only highly probable.

Now consider the possibility that first somehow we intuit the probability of one object being conjoined to something else, and from that intuition we conclude they are causally related necessarily. We would have no grounds however to be sure the connection is necessary merely from the basis that it is probable. The Earth's rotation is slowing. It is conceivable that like the moon, the earth one day will have one side of it that never faces inward toward the center of its orbit (the sun). So there might someday be "a dark side of the earth." In other words, it is not necessary that the sun rise every morning. Thus] we cannot conclude a necessity from a probability.

Yet, we did see that we can conlude a probability from a necessity. So probability inferences result from causal relations, not the other way around:

The same principle cannot be both the, cause and effect of another; and this is, perhaps, the only proposition concerning that relation, which is either intuitively or demonstratively certain. (90b)

Now, when we conclude that the sun will arise next morning, we base our inference on evidence, and we reason the effect will ensue. So we use our reasoning to arrive at probability conclusions. But inferences of cause to effect based on constant conjunction might be merely a passive association. Hume will further refute the argument that the inference is active and rational.

§211 Our Power to Infer Effects We Derive from the Cause's Power to Produce Them

So some philosophers argue that our inferences from cause to effect are based on our conducting solid reasoning. We have good rational reason to infer the effect from the cause. Hume challenges these philosophers. He dares them to exhibit exactly that causal reasoning so we may examine it.

They might make the following presentation:

1) We experience the constant conjunction of certain objects.

2) We find that one object always produces another.

3) The only way that a cause could produce an effect is if it had the power of production.

4) So it is the productive power of the cause that implies the effect. We need only conceive the cause's productive power to rationally infer its effect.

The power necessarily implies the effect; and therefore there is a just foundation for drawing a conclusion from the existence of one object to that of its usual attendant. The past production implies a power: The power implies a new production: And the new production is what we infer from the power and the past production. (90c)

§212 Hume will Present New Reasoning for his Refutation of Causal Reasoning

Hume could use his previous arguments to refute this thesis that we infer causal relations from powers of production. For example, he could show the notion of production is the same as causation; hence this argument circular or tautalogical. Also, Hume has shown that there are no properties in any object that imply a causal relation to any other object. Hence we cannot conceive of one object as having some power that implies that some other object will ensue.

But Hume will not weaken his presentation by making its different parts rely too much on each other. So he will instead perform a reductio argument on the proposition he wants to counter-argue.

§213 Unproductive Causation

Hume will first suppose their argument: a cause produces its effect because it has a productive power.

We know already that we cannot find any property in an object that implies some other object as an effect. So this power cannot be found in the cause's sensible qualities. But that means we need to find some other way to show that the object has this power. We might propose that past experience shows that the object always had this power. For it always accompanied its effect. But we cannot know from past experience that this conjunction of cause and effect will continue every time in the future. It is always possible that we might someday experience the one object without also experiencing the other.

§214 We cannot Rationally Know Necessary Connections. Only can We Suppose Them

So it is not by means of our reason alone that we discover the necessary connection between cause and effect. Yet experience might inform our reason that two objects have thusfar always been constantly connected. Nonetheless, we see that still we cannot rationally conclude the necessity of their connection on this basis.

We suppose, but are never able to prove, that there must be a resemblance betwixt those objects, of which we have had experience, and those which lie beyond the reach of our discovery. (91-92)

§215 The Fancy First Associates Ideas Causally, Then Reason Later Infers Effects from Causes

We have already noted that relations of succession, contiguity, and constant connection lead us to pass from one object to another, "even though there be no reason to determine us to that transition." (92a)

this we may establish for a general rule, that wherever the mind constantly and uniformly makes a transition without any reason, it is influenced by these relations. (92b)

This is precisely what happens in causal associations. If our reason does not make these associations actively, then our imagination makes them passively.

When the mind, therefore, passes from the idea or impression of one object to the idea or belief of another, it is not determin'd by reason, but by certain principles, which associate together the ideas of these objects, and unite them in the imagination. (92b)

We saw already that we have no rational grounds to infer effects from causes. Hence our imagination must first associate ideas causally so that we may later make these inferences.

Had ideas no more union in the fancy than objects seem to have to the understanding, we coued never draw any inference from causes to effects, nor repose belief in any matter of fact. The inference, therefore, depends solely on the union of ideas. (92bc)

§216 We Unite Ideas through Relations of Resemblance, Contiguity, and Causal Connection

So our imagination unites ideas. There are three principle unions among ideas which lead one idea to follow from some impression. Each of the three uniting principles has a relation as its basis. The three relations are:

a) resemblance,

b) contiguity, and

c) connection.

But, we may focus our attention on an object that we consider to be a cause, without also thereby conceiving its effect. So these principles do not infallibly bring about causal associations.

Also, our minds are perfectly capable of jumping between ideas that are not so closely related. Thus these relations are not the sole ways we make inferential leaps.

They are not the sole causes. For the thought has evidently a very irregular motion in running along its objects, and may leap from the heavens to the earth, from one end of the creation to the other, without any certain method or order. (92d)

But despite these imperfections, Hume maintains that resemblance, contiguity, and causation are the only general principles that we use to assiate ideas.

§217 Necessary Connections of Word Associations

But imagine also we hear someone utter, "blood." Our imaginations might automatically produce associated images. This move from sound to idea might seem to be a different principle of union between ideas.

However, the sound of a word is also an "object" of our impressions. So just like anything else we see or hear, word-sounds can prove to be constantly conjoined to some other object. Thus the associations we make from words come about by the same relations as any other union of ideas and impressions.

§218 The Cause of Our Beliefs

So our minds make associative leaps from ideas related by contiguity, succession, and constant connection. The principle behind these association is the same that leads us to infer effects from causes. This principle also provides the foundation for all our reasonings based on this relation.

We have no other notion of cause and effect, but that of certain objects, which have been always conjoin'd together, and which in all past instances have been found inseparable. (93c)

There could be a reason for why these conjunctions happen in the imagination. But any such reason is beyond our capacities to understand. Although, we can at least note that the associations are immediate. And thus they help explain how we come to have an opinion or belief.

We cannot penetrate into the reason of the conjunction. We only observe the thing itself, and always find that from the constant conjunction the objects acquire an union in the imagination. When the impression of one becomes present to us, we immediately form an idea of its usual attendant; and consequently we may establish this as one part of the definition of an opinion or belief, that it is an idea related to or associated with a present impression. (93c)

§219 Passive Basis for Active Reasoning

So causation implies contiguity, succession, and constant conjunction. This allows us to reason upon the relations. Hence causation is a "philosophical relation." But we are only able to reason upon it because it is a natural relation that [passively] unites ideas in our minds.

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From the original text:

Sect. vi. Of the Inference From the Impression to the Idea.
It is easy to observe, that in tracing this relation, the inference we draw from cause to effect, is not derived merely from a survey of these particular objects, and from such a penetration into their essences as may discover the dependance of the one upon the other. There is no object, which implies the existence of any other if we consider these objects in themselves, and never look beyond the ideas which we form of them. Such an inference would amount to knowledge, and would imply the absolute contradiction and impossibility of conceiving any thing different. But as all distinct ideas are separable, it is evident there can be no impossibility of that kind. When we pass from a present impression to the idea of any object, we might possibly have separated the idea from the impression, and have substituted any other idea in its room.
It is therefore by EXPERIENCE only, that we can infer the existence of one object from that of another. The nature of experience is this. We remember to have had frequent instances of the existence of one species of objects; and also remember, that the individuals of another species of objects have always attended them, and have existed in a regular order of contiguity and succession with regard to them. Thus we remember, to have seen that species of object we call flame, and to have felt that species of sensation we call heat. We likewise call to mind their constant conjunction in all past instances. Without any farther ceremony, we call the one cause and the other effect, and infer the existence of the one from that of the other. In all those instances, from which we learn the conjunction of particular causes and effects, both the causes and effects have been perceived by the senses, and are remembered But in all cases, wherein we reason concerning them, there is only one perceived or remembered, and the other is supplyed in conformity to our past experience.
Thus in advancing we have insensibly discovered a new relation betwixt cause and effect, when we least expected it, and were entirely employed upon another subject. This relation is their CONSTANT CONJUNCTION. Contiguity and succession are not sufficient to make us pronounce any two objects to be cause and effect, unless we perceive, that these two relations are preserved in several instances. We may now see the advantage of quitting the direct survey of this relation, in order to discover the nature of that necessary connexion, which makes so essential a part of it. There are hopes, that by this means we may at last arrive at our proposed end; though to tell the truth, this new-discovered relation of a constant conjunction seems to advance us but very little in our way. For it implies no more than this, that like objects have always been placed in like relations of contiguity and succession; and it seems evident, at least at first sight, that by this means we can never discover any new idea, and can only multiply, but not enlarge the objects of our mind. It may be thought, that what we learn not from one object, we can never learn from a hundred, which are all of the same kind, and are perfectly resembling in every circumstance. As our senses shew us in one instance two bodies, or motions, or qualities in certain relations of success and contiguity; so our memory presents us only with a multitude of instances, wherein we always find like bodies, motions, or qualities in like relations. From the mere repetition of any past impression, even to infinity, there never will arise any new original idea, such as that of a necessary connexion; and the number of impressions has in this case no more effect than if we confined ourselves to one only. But though this reasoning seems just and obvious; yet as it would be folly to despair too soon, we shall continue the thread of our discourse; and having found, that after the discovery of the constant conjunction of any objects, we always draw an inference from one object to another, we shall now examine the nature of that inference, and of the transition from the impression to the idea. Perhaps it will appear in the end, that the necessary connexion depends on the inference, instead of the inference’s depending on the necessary connexion.
Since it appears, that the transition from an impression present to the memory or senses to the idea of an object, which we call cause or effect, is founded on past experience, and on our remembrance of their constant conjunction, the next question is, Whether experience produces the idea by means of the understanding or imagination; whether we are determined by reason to make the transition, or by a certain association and relation of perceptions. If reason determined us, it would proceed upon that principle, that instances, of which we have had no experience, must resemble those, of which we have had experience, and that the course of nature continues always uniformly the same. In order therefore to clear up this matter, let us consider all the arguments, upon which such a proposition may be supposed to be founded; and as these must be derived either from knowledge or probability, let us cast our eve on each of these degrees of evidence, and see whether they afford any just conclusion of this nature.
Our foregoing method of reasoning will easily convince us, that there can be no demonstrative arguments to prove, that those instances, of which we have, had no experience, resemble those, of which we have had experience. We can at least conceive a change in the course of nature; which sufficiently proves, that such a change is not absolutely impossible. To form a clear idea of any thing, is an undeniable argument for its possibility, and is alone a refutation of any pretended demonstration against it.
Probability, as it discovers not the relations of ideas, considered as such, but only those of objects, must in some respects be founded on the impressions of our memory and senses, and in some respects on our ideas. Were there no mixture of any impression in our probable reasonings, the conclusion would be entirely chimerical: And were there no mixture of ideas, the action of the mind, in observing the relation, would, properly speaking, be sensation, not reasoning. It is therefore necessary, that in all probable reasonings there be something present to the mind, either seen or remembered; and that from this we infer something connected with it, which is not seen nor remembered.
The only connexion or relation of objects, which can lead us beyond the immediate impressions of our memory and senses, is that of cause and effect; and that because it is the only one, on which we can found a just inference from one object to another. The idea of cause and effect is derived from experience, which informs us, that such particular objects, in all past instances, have been constantly conjoined with each other: And as an object similar to one of these is supposed to be immediately present in its impression, we thence presume on the existence of one similar to its usual attendant. According to this account of things, which is, I think, in every point unquestionable, probability is founded on the presumption of a resemblance betwixt those objects, of which we have had experience, and those, of which we have had none; and therefore it is impossible this presumption can arise from probability. The same principle cannot be both the, cause and effect of another; and this is, perhaps, the only proposition concerning that relation, which is either intuitively or demonstratively certain.
Should any one think to elude this argument; and without determining whether our reasoning on this subject be derived from demonstration or probability, pretend that all conclusions from causes and effects are built on solid reasoning: I can only desire, that this reasoning may be produced, in order to be exposed to our examination. It may, perhaps, be said, that after experience of the constant conjunction of certain objects, we reason in the following manner. Such an object is always found to produce another. It is impossible it coued have this effect, if it was not endowed with a power of production. The power necessarily implies the effect; and therefore there is a just foundation for drawing a conclusion from the existence of one object to that of its usual attendant. The past production implies a power: The power implies a new production: And the new production is what we infer from the power and the past production.
It were easy for me to shew the weakness of this reasoning, were I willing to make use of those observations, I have already made, that the idea of production is the same with that of causation, and that no existence certainly and demonstratively implies a power in any other object; or were it proper to anticipate what I shall have occasion to remark afterwards concerning the idea we form of power and efficacy. But as such a method of proceeding may seem either to weaken my system, by resting one part of it on another, or to breed a confusion in my reasoning, I shall endeavour to maintain my present assertion without any such assistance.
It shall therefore be allowed for a moment, that the production of one object by another in any one instance implies a power; and that this power is connected with its effect. But it having been already proved, that the power lies not in the sensible qualities of the cause; and there being nothing but the sensible qualities present to us; I ask, why in other instances you presume that the same power still exists, merely upon the appearance of these qualities? Your appeal to past experience decides nothing in the present case; and at the utmost can only prove, that that very object, which produced any other, was at that very instant endowed with such a power; but can never prove, that the same power must continue in the same object or collection of sensible qualities; much less, that a like power is always conjoined with like sensible qualities. Should it be said, that we have experience, that the same power continues united with the same object, and that like objects are endowed with like powers, I would renew my question, why from this experience we form any conclusion beyond those past instances, of which we have had experience. If you answer this question in, the same manner as the preceding, your answer gives still occasion to a new question of the same kind, even in infinitum; which clearly proves, that the foregoing reasoning had no just foundation.
Thus not only our reason fails us in the discovery of the ultimate connexion of causes and effects, but even after experience has informed us of their constant conjunction, it is impossible for us to satisfy ourselves by our reason, why we should extend that experience beyond those particular instances, which have fallen under our observation. We suppose, but are never able to prove, that there must be a resemblance betwixt those objects, of which we have had experience, and those which lie beyond the reach of our discovery.
We have already taken notice of certain relations, which make us pass from one object to another, even though there be no reason to determine us to that transition; and this we may establish for a general rule, that wherever the mind constantly and uniformly makes a transition without any reason, it is influenced by these relations. Now this is exactly the present case. Reason can never shew us the connexion of one object with another, though aided by experience, and the observation of their constant conjunction in all past instances. When the mind, therefore, passes from the idea or impression of one object to the idea or belief of another, it is not determined by reason, but by certain principles, which associate together the ideas of these objects, and unite them in the imagination. Had ideas no more union in the fancy than objects seem to have to the understanding, we coued never draw any inference from causes to effects, nor repose belief in any matter of fact. The inference, therefore, depends solely on the union of ideas.
The principles of union among ideas, I have reduced to three general ones, and have asserted, that the idea or impression of any object naturally introduces the idea of any other object, that is resembling, contiguous to, or connected with it. These principles I allow to be neither the infallible nor the sole causes of an union among ideas. They are not the infallible causes. For one may fix his attention during Sometime on any one object without looking farther. They are not the sole causes. For the thought has evidently a very irregular motion in running along its objects, and may leap from the heavens to the earth, from one end of the creation to the other, without any certain method or order. But though I allow this weakness in these three relations, and this irregularity in the imagination; yet I assert that the only general principles, which associate ideas, are resemblance, contiguity and causation.
There is indeed a principle of union among ideas, which at first sight may be esteemed different from any of these, but will be found at the bottom to depend on the same origin. When every individual of any species of objects is found by experience to be constantly united with an individual of another species, the appearance of any new individual of either species naturally conveys the thought to its usual attendant. Thus because such a particular idea is commonly annexed to such a particular word, nothing is required but the hearing of that word to produce the correspondent idea; and it will scarce be possible for the mind, by its utmost efforts, to prevent that transition. In this case it is not absolutely necessary, that upon hearing such a particular sound we should reflect on any past experience, and consider what idea has been usually connected with the sound. The imagination of itself supplies the place of this reflection, and is so accustomed to pass from the word to the idea, that it interposes not a moment’s delay betwixt the hearing of the one, and the conception of the other.
But though I acknowledge this to be a true principle of association among ideas, I assert it to be the very same with that betwixt the ideas of cause and effects and to be an essential part in all our reasonings from that relation. We have no other notion of cause and effect, but that of certain objects, which have been always conjoined together, and which in all past instances have been found inseparable. We cannot penetrate into the reason of the conjunction. We only observe the thing itself, and always find that from the constant conjunction the objects acquire an union in the imagination. When the impression of one becomes present to us, we immediately form an idea of its usual attendant; and consequently we may establish this as one part of the definition of an opinion or belief, that it is an idea related to or associated with a present impression.
Thus though causation be a philosophical relation, as implying contiguity, succession, and constant conjunction, yet it is only so far as it is a natural relation, and produces an union among our ideas, that we are able to reason upon it, or draw any inference from it.