The Uniformity Principle

More notes for 13 January

Two questions

In, we are told that there are two possible explanations of how we make particular causal inferences: either “we are determined by reason to make the transition, or by a certain association and relation of perceptions.” (“the transition” is from an impression to a forceful idea [a belief] in the impression’s cause or one of its effects.)

Then we are told that “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.” I have been calling this the Uniformity Principle.

In order to discover whether this is so, we are told to “consider all the arguments upon which such a proposition may be supposed to be founded.” The two kinds of arguments are demonstrations and probabilistic arguments and, of course, both fail to found the Uniformity Principle.

Here is a question raised by Will and Aidan today: why wouldn’t it be enough that we believe the Uniformity Principle? After all, the Uniformity Principle holds that the things we have not experienced must resemble those we have experienced and that nature continues always uniformly the same. Suppose I had experience of As constantly followed by Bs, an impression of an A, and that I believed that things must continue as they had in the past. Wouldn’t I be able to conclude that B would follow by comparing my ideas?

I had said that we do not, in fact, use the Uniformity Principle in drawing causal inferences. But Will asked why that is so obvious. Couldn’t it be the case that we tacitly believe the Uniformity Principle without consciously thinking of it?

So here’s what we have.

Question A
Why must we have a founding argument for the Uniformity Principle; wouldn’t belief in the Uniformity Principle be enough to explain why we make causal inferences using reason?
Question B
Why must belief in the Uniformity Principle be explicit, conscious belief; why can’t it be tacit, unconscious belief?

I will start with Question B; really, you should just pop open your book to pp. 72-3 (; that’s what the big quotations are from.

Question B

The second paragraph ( claims that we tacitly believe the Uniformity Principle and that we have “many millions” of experiments to confirm it. That seems to mean that the answer to Question B is: we can tacitly believe the Uniformity Principle and, in fact, we do tacitly believe the Uniformity Principle.

The first paragraph ( claims that we often make causal inferences without explicitly thinking of the Uniformity Principle. And it claims that this shows that we do not use reason to make our causal inferences. That seems to mean that we cannot extend the affirmative answer to question B to save the reason explanation of causal inferences: the reason explanation of causal inferences works only if our knowledge and use of the Uniformity Principle is explicit.

It will here be worth our observation, that the past experience, on which all our judgments concerning cause and effect depend, may operate on our mind in such an insensible manner as never to be taken notice of, and may even in some measure be unknown to us. A person, who stops short in his journey upon meeting a river in his way, foresees the consequences of his proceeding forward; and his knowledge of these consequences is conveyed to him by past experience, which informs him of such certain conjunctions of causes and effects. But can we think, that on this occasion he reflects on any past experience, and calls to remembrance instances that he has seen or heard of, in order to discover the effects of water on animal bodies? No, surely; this is not the method, in which he proceeds in his reasoning. The idea of sinking is so closely connected with that of water, and the idea of suffocating with that of sinking, that the mind makes the transition without the assistance of the memory. The custom operates before we have time for reflection. The objects seem so inseparable, that we interpose not a moment’s delay in passing from the one to the other. But as this transition proceeds from experience, and not from any primary connexion betwixt the ideas, we must necessarily acknowledge, that experience may produce a belief and a judgment of causes and effects by a secret operation, and without being once thought of. This removes all pretext, if there yet remains any, for asserting that the mind is convinced by reasoning of that principle, that instances of which we have no experience, must necessarily resemble those of which we have. For we here find, that the understanding or imagination can draw inferences from past experience, without reflecting on it; much more without forming any principle concerning it, or reasoning upon that principle.

In general we may observe, that in all the most established and uniform conjunctions of causes and effects, such as those of gravity, impulse, solidity, etc., the mind never carries its view expressly to consider any past experience: though in other associations of objects, which are more rare and unusual, it may assist the custom and transition of ideas by this reflection. Nay, we find in some cases, that the reflection produces the belief without the custom; or, more properly speaking, that the reflection produces the custom in an oblique and artificial manner. I explain myself. It is certain, that not only in philosophy, but even in common life, we may attain the knowledge of a particular cause merely by one experiment, provided it be made with judgment, and after a careful removal of all foreign and superfluous circumstances. Now, as after one experiment of this kind, the mind, upon the appearance either of the cause or the effect, can draw an inference concerning the existence of its correlative, and as a habit can never be acquired merely by one instance, it may be thought that belief cannot in this case be esteemed the effect of custom. But this difficulty will vanish, if we consider, that, though we are here supposed to have had only one experiment of a particular effect, yet we have many millions to convince us of this principle, that like objects, placed in like circumstances, will always produce like effects; and as this principle has established itself by a sufficient custom, it bestows an evidence and firmness on any opinion to which it can be applied. The connexion of the ideas is not habitual after one experiment; but this connexion is comprehended under another principle that is habitual; which brings us back to our hypothesis. In all cases we transfer our experience to instances of which we have no experience, either expressly or tacitly, either directly or indirectly.

Question A

But why must we found the Uniformity Principle, that is, why do we have to give arguments in support of it? Why isn’t it enough that we believe in the Uniformity Principle, as the second paragraph says we do, no matter why we believe in it? After all, we don’t ask why we believe our impressions, we don’t demand arguments showing that having an impression of an A gives us certainty that there was an A before we consider drawing causal inferences from that impression.

And, if it’s enough to believe in the Uniformity Principle, why must the belief be explicit?

What I think

I can think of four things to say.

First, I am unsure just what role Hume thinks the Uniformity Principle plays in our drawing causal inferences. The first paragraph suggests we don’t use it at all in many cases, the second paragraph suggests we use it quite a lot, albeit tacitly.

Strictly speaking, those are compatible claims. But they fit together oddly.

Second, either way, there appears to be an asymmetry between the association explanation and the reason explanation. Hume held that the reason explanation has to involve the conscious invocation of the Uniformity Principle when we make causal inferences. But he does not seem to have held that the same is true of the association explanation: see the second paragraph (

Is this sort of asymmetry explicitly acknowledged anywhere? I don’t know.

Third, it’s possible that he thought that our causal inferences could be the products of reason only if two things are true: (a) we use the Uniformity Principle in making them and (b) we are certain of the truth of the Uniformity Principle itself.

I am unsure about this. Looking carefully at 1.3.6, I don’t see an assertion of (b). Instead, both demonstrative and probabilistic arguments are treated as possible foundations for the Uniformity Principle. On the other hand, he seems to say that there is a pretty good probabilistic argument for the Uniformity Principle in 1.3.14: millions of experiments confirm it. If that isn’t enough, perhaps it’s because he required certainty, as in (b). If so, he did not explain why he found (b) to be an obvious requirement. Perhaps it has something to do with obtaining a certain result: granted the Uniformity Principle is about how things must be, but if we are not certain that it is true, we cannot be certain about how things must be and so we cannot make certain inferences based on what the Uniformity Principle says must happen. That's just a guess on my part, though.

Fourth, all of this may well be more evidence that Hume did not really think carefully about the reason account as a possible explanation of how we make causal inferences. In 1.3.6, he may have been more interested in discussing the question of our justification for drawing causal inferences than in the explanation of how we do so. Again, if he were interested in explanations, it’s hard to see why belief in the Uniformity Principle, tacit or explicit, would not do the trick.