Pirates & Revolutionaries: Hume, A Treatise of Human Nature, Book 1, Part 3, Sect 2 "Of Probability; and of the Idea of Cause and Effect" §§168-183

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 II: Of Probability; and of the Idea of Cause and Effect

§168 Less Certain Relations

Previously we saw that there are four relations between ideas that we may intuit with certainty. And hence they provide the foundations for any science. Those relations are: resemblance, contrariety, degrees in quality, and proportions in quantity or number. This leaves three others that do not depend on the idea being related: identity, situations in time and place, and causation.

§169 How Causation is Identified and How Identity is Caused

Almost every time we reason, we are making a comparison. We thereby discover the relations that two ideas share. Either, both, or none of the relations need be present to our senses for us to relate their ideas. But when both are sensibly presented along with their relation, our mind passively admits the impressions through the sense organs. Hence in these cases our comparison is a perception rather than reasoning. So which types of relations are obtained through perception?

Whether or not things are identical is something only our senses tell us. And as well, we depend on our senses to tell us when objects are related by time and place.

But consider when we always see certain things together. Or when we never see them together. There is nothing immediately given in each object that indicates this relation. And if their remoteness or contiguity is consistently so, we tend to presume there is some cause or reason.

when from experience and observation we discover, that their relation in this particular is invariable, we, always conclude there is some secret cause, which separates or unites them. (74a)

Consider a similar instance. We see something at one time, then come back to see it there still later. There is a series of perceptions spanning between the two appearances of the object. But nonetheless we ascribe to it an identity that presumably held even while we were not perceiving it. For we seem to presume that "if we had kept our eye or hand constantly upon it, it would have conveyed an invariable and uninterrupted perception." (74b) [see §151 for Hume's description of how this leads to the mistaken notion of time without changing objects.] But to conclude that something remains identical is to go beyond what our senses tell us. For we never perceived the object's invariance in-between its different appearances.

So we see a statue at five o'clock. We return at six to observe it has not changed, except perhaps the sun's new position has created differences in the shadows that the statue casts. Then we wonder if all statues continue to bear a resemblance to themselves in this way. And we conclude that yes, they all do, despite each being a different statue. Then we consider what could cause the statue to remain the same, and what explains the slight variations in its shadows. We speculate that the statue's stone material is highly durable, and that the sun is a different entity altogether. So the changes in shadow are caused by a variance external to the statue, and are not inherent to it. We also assess that the resemblances between the statue's appearances could not have resulted from it being replaced by a replica. For that would be a difficult feat, and there is no evidence to suggest it was accomplished. We then judge that the statue we see at six is identically the same one that we saw before at five. [Review Hume's original passage if an alternate interpretation is desired: "Whenever we discover such a perfect resemblance, we consider, whether it be common in that species of objects; whether possibly or probably any cause cou'd operate in producing the change and resemblance; and according as we determine concerning these causes and effects, we form our judgment concerning the identity of the object" (74c).]

§170 Causation beyond Sensation

So we noted the three relations that do not depend on the ideas being related: identity, situations in time and place, and causation. Of these, only causation may be traced beyond our senses and inform us of existences and objects that we do not see or feel. Hume will now examine causal relations.

§171 The Origin of Causation

(or, What is Causation's Cause?)

We need to perfectly understand the ideas we reason-with, if we want our reasoning to be just and adequate. But, we cannot obtain a perfect understanding of an idea if we do not trace it back to its origin. We know that ideas originate in impressions. So to trace an idea to its origin is to uncover the primary impression that it arises-from. By doing so, the idea and our reasoning on it obtains clarity:

The examination of the impression bestows a clearness on the idea; and the examination of the idea bestows a like clearness on all our reasoning. (75a)

Now, we want to understand the idea of causation, so we must trace it back to its impressional origins.

§172 Everything is a Specific Example of Cause and Effect

To find causation's origin, we consider a concrete instance of a cause and an effect. We know that the relation of causation is not an inherent quality of any object. Yet we find that every object we have ever encountered has already obtained the relation of cause or effect. And yet, we also know that there is no universal quality shared by all things that explains why they all are already causally related.

§173 Contiguity and Causation

So rather than find causation as a quality in any one object, we must find causality as relation between things.

Hume notes firstly that in all cases where objects are related causally, they are to be found contiguously in time and space. And even distant objects that seem causally related are really linked by a chain of intermediate contiguous causes. In fact, even if we cannot determine some complex chain of causes between distant objects, we still presume there must be one, in order for them to be causally related.

We may therefore consider the relation of contiguity as essential to that of causation. (75d)

§174 Causation is Temporal

Besides contiguity, Hume recognizes another relation found between cause and effect, although it is not universally acknowledged. Hume argues that the cause must precede the effect temporally. Hence this second relation is the priority of time in the cause before the effect. Some philosophers argue that causes can occur simultaneously with their effects. Hume firstly says that experiences tell us otherwise. He then proceeds to conclude this by inference.

Consider first when cause and effect are not contemporaneous. We strike a match. The chemicals reacting to cause the flame are consumed in the process. The match-tip causes the fire, but it is not present at the same time as the flame. But we are wondering instead about when cause and effect are found together at the same time. The match-tip did not remain in its perfect state throughout its relating to the flame as its cause. But a cause contemporaneous with its effect would remain in its perfect state.

Consider first a case when cause and effect are not contemporaneous. We strike a match. The fire consumes the match-tip, whose chemicals helped bring about the flame. So the match-tip and the fire it causes cannot be contemporaneous. However, note also that the box of matches stays safely unlit in its box until we strike it against a rough surface. The match-tip remains in its state of inactivity until acted-on by another causal principle, its action of being struck. Then, the tip is pushed from its inactive state, and is forced to manifest the energy it stored-up. So because the tip did not cause fire until being struck meant that it could not be the sole cause of the flame. Hume says then, that the following is a maxim in both natural and moral philosophy:

an object, which exists for any time in its full perfection without producing another, is not its sole cause; but is assisted by some other principle, which pushes it from its state of inactivity, and makes it exert that energy, of which it was secretly possest. (76b)

To strike the match, we must pick up the match, aim it to the match-box's rough side, gather muscle tension, release it, swinging the match through the air, to the sand-paper, brushing it across, and withdrawing a quickly flaring lit match. Now, if the match-head is contemporaneous with the flame, we have not explained how something inactive like the match-tip brought about that flame. For, there is no sequence of events to show that one led to the other. Hence the cause cannot be discerned from its effect in this case. We need also to account for the force acting on the match so that it may raise the flame. So we need to make the arm-motions part of that simultaneity. But it is inconceivable how our hands can both be aiming the match in the air, swinging it across the rough paper, and withdrawing the lit match. So it cannot be that any one cause is contemporaneous with its effect. For then, we would need to include that cause's cause, and maybe even those causes' causes, and so on. But every experience we have of causation would say this cannot ever be.

The consequence of this would be no less than the destruction of that succession of causes, which we observe in the world; and indeed, the utter annihilation of time. For if one cause were co-temporary with its effect, and this effect with its effect, and so on, it is plain there would be no such thing as succession, and all objects must be co-existent. (76c)

§175 Let's Just Agree that Causation is Temporal

Hume argued above that causes can never coincide with their effects. He admits however that some people might not be satisfied with his proof. Yet, the matter is not so consequential in the whole of his argument. So he says either forget it, or go along with his claim. Both are fine by him.

§176 The Only Relations in Causation are Succession and Contiguity

Imagine billiard cue-ball approaching and striking the 8-ball, which flies off its own way. We see that the cue-ball's motion preceded the 8-ball's. And when we examine the moment of contact, we do not notice any interval between the two motions. For, it seems that the 8-ball moves immediately after the cue hits it. Hume says that in such instances of causation there are no more relations to observe than contiguity and succession.

§177 Causation is not Reducible to Production Only

Some might instead define causation as production. But Hume asks such people to explain production without involving the idea of causation. For, any attempt to do so will lead to circular reasoning.

§178 Contiguity and Succession are Necessary but not Sufficient Conditions.

We also Need 'Necessary Connection'

So contiguity and succession are the only relations involved in causation. Now consider the piece of felt under the 8-ball before the cue-ball impacts it. That fabric-piece is contiguous to the 8-ball. As well, its being under the 8-ball precedes the successive movement away. But, we consider the cue-ball as the 8-ball's cause of motion. The felt underneath it never pushed the ball on its own. And there are countless similar cases where two things related successively and contiguously are not causally related. So there is more to causation than contiguity and succession. Thus if we only take these two relations into account, we do not obtain a complete idea of causality. The table-top does not necessarily cause the ball to move. However, when another object impacts it and transfers enough force to set it in motion, that will necessarily cause it to move away. Hence a NECESSARY CONNEXION between causally related-objects is an essential factor determining which objects are causally related.

§179 With no Other Explanation, We seem Compelled to Consider the Necessary Connection between Cause and Effect as an A Priori Idea

Hume will now examine causally-related objects to see which impression leads to the idea of necessary connection.

Note firstly that we may determine all the qualities of causally related things. But we will find that their causal relation depends on none of their attributes. And when we consider their relations, all we find are contiguity and succession.

Now recall that all ideas are derived from impressions [§17]. But so far we cannot find any impression that explains the necessary connection between causes and their effects. Hume, however, is not ready to conclude that we have an inborn notion of 'necessary connection,' as an a priori idea.

§180 We are Forced to Look to Related Questions for Clues

Often when we cannot find something where we expect it, we dig about in the surrounding regions hoping that by chance we discover what we seek. Hume then will detour briefly from the analysis of the necessary connection between cause and effect. Instead, he will address two other questions that might provide some clues.

§181 Question 1: Why are Beginnings Caused?

We ask firstly: why do we deem it necessary that all things with beginnings must as well have causes?

§182 Question 2: What Causes Us to See Causation this Way?

We ask secondly:

a) why do we conclude that certain causes must necessarily produce certain other effects? And

b) we base the inference of cause to its effect on some reasoning. What is this basis? Also, we have certain beliefs about causation that compel us to make these inferences. So what are those beliefs?

§183 Passions may Bear Causal Relations

In the following section, Hume will address these questions in detail. For now, Hume has us consider something.

First, imagine a child marvel at the pleasant warm glow of a red-hot fire-poker. He reaches for it. So he burns his hands. He has a sense impression of its heat. Thereby he obtains an idea of heat. Next time he sees something that is red-hot, the child recalls getting burned. This has an effect on him. He then shows aversion to hot things.

So first we have sense impressions. The mind copies these impressions, thereby forming ideas. We then recall these ideas. They then give us new impressions, called impressions of reflections [see §24 for more on this process].

Hume wants us to note that so far he has only spoken of how we come to grasp causal relations by means of sense impressions alone. However, he also wants to extend his observations to impressions of reflections as well. Recall that the burned child's reflection of an impression was his aversion to extremely hot things. This aversion is a passion, which is the connection between

a) the idea of getting burned, and

b) the impression he obtains when recalling that idea.

His aversion is a very strong relation between him and red-hot objects. Hume wants us to consider the causal relation as also holding for our passions as well.

Passions are connected with their objects and with one another; no less than external bodies are connected together. The same relation, then, of cause and effect, which belongs to one, must be common to all of them. (78c)

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From the original text:

Sect. ii. Of Probability, and of the Idea of Cause and Effect.
This is all I think necessary to observe concerning those four relations, which are the foundation of science; but as to the other three, which depend not upon the idea, and may be absent or present even while that remains the same, it will be proper to explain them more particularly. These three relations are identity, the situations in time and place, and causation.
All kinds of reasoning consist in nothing but a comparison, and a discovery of those relations, either constant or inconstant, which two or more objects bear to each other. This comparison we may make, either when both the objects are present to the senses, or when neither of them is present, or when only one. When both the objects are present to the senses along with the relation, we call this perception rather than reasoning; nor is there in this case any exercise of the thought, or any action, properly speaking, but a mere passive admission of the impressions through the organs of sensation. According to this way of thinking, we ought not to receive as reasoning any of the observations we may make concerning identity, and the relations of time and .place; since in none of them the mind can go beyond what is immediately present to the senses, either to discover the real existence or the relations of objects. It is only causation, which produces such a connexion, as to give us assurance from the existence or action of one object, that it was followed or preceded by any other existence or action; nor can the other two relations be ever made use of in reasoning, except so far as they either affect or are affected by it. There is nothing in any objects to perswade us, that they are either always remote or always contiguous; and when from experience and observation we discover, that their relation in this particular is invariable, we, always conclude there is some secret cause, which separates or unites them. The same reasoning extends to identity. We readily suppose an object may continue individually the same, though several times absent from and present to the senses; and ascribe to it an identity, notwithstanding the interruption of the perception, whenever we conclude, that if we had kept our eye or hand constantly upon it, it would have conveyed an invariable and uninterrupted perception. But this conclusion beyond the impressions of our senses can be founded only on the connexion of cause and effect; nor can we otherwise have any security, that the object is not changed upon us, however much the new object may resemble that which was formerly present to the senses. Whenever we discover such a perfect resemblance, we consider, whether it be common in that species of objects; whether possibly or probably any cause coued operate in producing the change and resemblance; and according as we determine concerning these causes and effects, we form our judgment concerning the identity of the object.
Here then it appears, that of those three relations, which depend not upon the mere ideas, the only one, that can be traced beyond our senses and informs us of existences and objects, which we do not see or feel, is causation. This relation, therefore, we shall endeavour to explain fully before we leave the subject of the understanding.
To begin regularly, we must consider the idea of causation, and see from what origin it is derived. It is impossible to reason justly, without understanding perfectly the idea concerning which we reason; and it is impossible perfectly to understand any idea, without tracing it up to its origin, and examining that primary impression, from which it arises. The examination of the impression bestows a clearness on the idea; and the examination of the idea bestows a like clearness on all our reasoning.
Let us therefore cast our eye on any two objects, which we call cause and effect, and turn them on all sides, in order to find that impression, which produces an idea, of such prodigious consequence. At first sight I perceive, that I must not search for it in any of the particular qualities of the objects; since which-ever of these qualities I pitch on, I find some object, that is not possessed of it, and yet falls under the denomination of cause or effect. And indeed there is nothing existent, either externally or internally, which is not to be considered either as a cause or an effect; though it is plain there is no one quality, which universally belongs to all beings, and gives them a title to that denomination.
The idea, then, of causation must be derived from some relation among objects; and that relation we must now endeavour to discover. I find in the first place, that whatever objects are considered as causes or effects, are contiguous; and that nothing can operate in a time or place, which is ever so little removed from those of its existence. Though distant objects may sometimes seem productive of each other, they are commonly found upon examination to be linked by a chain of causes, which are contiguous among themselves, and to the distant objects; and when in any particular instance we cannot discover this connexion, we still presume it to exist. We may therefore consider the relation of CONTIGUITY as essential to that of causation; at least may suppose it such, according to the general opinion, till we can find a more [Part IV. Sect. 5.] proper occasion to clear up this matter, by examining what objects are or are not susceptible of juxtaposition and conjunction.
The second relation I shall observe as essential to causes and effects, is not so universally acknowledged, but is liable to some controversy. It is that of PRIORITY Of time in the cause before the effect. Some pretend that it is not absolutely necessary a cause should precede its effect; but that any object or action, in the very first moment of its existence, may exert its productive quality, and give rise to another object or action, perfectly co-temporary with itself. But beside that experience in most instances seems to contradict this opinion, we may establish the relation of priority by a kind of inference or reasoning. It is an established maxim both in natural and moral philosophy, that an object, which exists for any time in its full perfection without producing another, is not its sole cause; but is assisted by some other principle, which pushes it from its state of inactivity, and makes it exert that energy, of which it was secretly possest. Now if any cause may be perfectly co-temporary with its effect, it is certain, according to this maxim, that they must all of them be so; since any one of them, which retards its operation for a single moment, exerts not itself at that very individual time, in which it might have operated; and therefore is no proper cause. The consequence of this would be no less than the destruction of that succession of causes, which we observe in the world; and indeed, the utter annihilation of time. For if one cause were co-temporary with its effect, and this effect with its effect, and so on, it is plain there would be no such thing as succession, and all objects must be co-existent.
If this argument appear satisfactory, it is well. If not, I beg the reader to allow me the same liberty, which I have used in the preceding case, of supposing it such. For he shall find, that the affair is of no great importance.
Having thus discovered or supposed the two relations of contiguity and succession to be essential to causes and effects, I find I am stopt short, and can proceed no farther in considering any single instance of cause and effect. Motion in one body is regarded upon impulse as the cause of motion in another. When we consider these objects with utmost attention, we find only that the one body approaches the other; and that the motion of it precedes that of the other, but without any, sensible interval. It is in vain to rack ourselves with farther thought and reflection upon this subject. We can go no farther in considering this particular instance.
Should any one leave this instance, and pretend to define a cause, by saying it is something productive of another, it is evident he would say nothing. For what does he mean by production? Can he give any definition of it, that will not be the same with that of causation? If he can; I desire it may be produced. If he cannot; he here runs in a circle, and gives a synonimous term instead of a definition.
Shall we then rest contented with these two relations of contiguity and succession, as affording a complete idea of causation? By, no means. An object may be contiguous and prior to another, without being considered as its cause. There is a NECESSARY CONNEXION to be taken into consideration; and that relation is of much greater importance, than any of the other two above-mentioned.
Here again I turn the object on all sides, in order to discover the nature of this necessary connexion, and find the impression, or impressions, from which its idea may be derived. When I cast my eye on the known Qualities of objects, I immediately discover that the relation of cause and effect depends not in the least on them. When I consider their relations, I can find none but those of contiguity and succession; which I have already regarded as imperfect and unsatisfactory. Shall the despair of success make me assert, that I am here possest of an idea, which is not preceded by any similar impression? This would be too strong a proof of levity and inconstancy; since the contrary principle has been already so firmly established, as to admit of no farther doubt; at least, till we have more fully examined the present difficulty.
We must, therefore, proceed like those, who being in search of any thing, that lies concealed from them, and not finding it in the place they expected, beat about all the neighbouring fields, without any certain view or design, in hopes their good fortune will at last guide them to what they search for. It is necessary for us to leave the direct survey of this question concerning the nature of that necessary connexion, which enters into our idea of cause and effect; and endeavour to find some other questions, the examination of which will perhaps afford a hint, that may serve to clear up the present difficulty. Of these questions there occur two, which I shall proceed to examine, viz.
First, For what reason we pronounce it necessary, that every thing whose existence has a beginning, should also have a cause.
Secondly, Why we conclude, that such particular causes must necessarily have such particular effects; and what is the nature of that inference we draw from the one to the other, and of the belief we repose in it?
I shall only observe before I proceed any farther, that though the ideas of cause and effect be derived from the impressions of reflection as well as from those of sensation, yet for brevity’s sake, I commonly mention only the latter as the origin of these ideas; though I desire that whatever I say of them may also extend to the former. Passions are connected with their objects and with one another; no less than external bodies are connected together. The same relation, then, of cause and effect, which belongs to one, must be common to all of them.