Hume and Causality

Hume calls causality "the cement of the universe" -- implying that it holds everything together. Causality is an important topic in philosophy generally and in the philosophy of science. Understanding what causes are helps us to understand how minds might (or might not) relate to bodies, how free will might (or might not) work, how ideas might (or might not) influence action, and how bodies might come to produce changes in other bodies.

The account of causality which one develops or accepts also plays an important part in one's view about the legitimacy of various philosophical activities. Some philosophers before Hume regarded propositions about causation as necessary truths, capable of being known a priori. Such philosophers claimed to know not only that everything has a cause, but, in particular cases that that this will cause that, or that this was caused by that. The realm of causality and the realm of logic are conflated or run together. Thus a philosopher like Spinoza thought of metaphysics as a deductive account of the universe to be developed from a few definitions -- particularly that of substance as something that requires nothing outside itself to be or to be conceieved, along with a number of self-evident assumptions. So, the old account of causality had these features:

  1. Propositions about causation are necessary truths.
  2. Propositions about particular causes can be known a priori or prior to experience.
  3. Propositions about particular causes involve logical as well as causal connections.
  4. Because of the nature of causality, rationalist metaphysics of the kind practiced by Spinoza is possible.

Hume's analysis of necessary connection

Concerning the ideas of force, power and necessary connection, Hume remarks:

49. There are no ideas, which occur in metaphysics, more obscure and uncertain, than those of power, force, energy or necessary connexion, of which it is every moment necessary for us to treat in all our disquisitions.
This makes these ideas, ideal candidates for the use of Hume's microscope. At the beginning of section 2 Hume says:
50. To be fully acquainted, therefore, with the idea of power or necessary connexion, let us examine its impression; and in order to find the impression with greater certainty, let us search for it in all the sources, from which it may possibly be derived.
Hume begins with a single instance of causality. All we observe is one billard bill hitting another and the second moving. "This is the whole that appears to the outward senses. The mind feels no sentiment or inward impression from this succession of objects: Consequently, there is not, in any single, particular instance of cause and effect, anything which can suggest the idea of power or necessary connexion."

There are really two possible sources from which the idea of necessary connection could be derived from sense impressions of external objects or from the impressions we have of the operations of our own minds. In either case the idea of necessary connection might be derived (a) from the cause, (b) from the effect, or (c) from the relation between the cause and the effect. Hume begins with the our impressions of external objects and remarks:

In reality, there is no part of matter, that does ever, by its sensible qualities, discover any power or energy, or give us ground to imagine, that it could produce any thing, or be followed by any other object, which we could denominate its effect. Solidity, extension, motion; these qualities are all complete in themselves, and never point out any other event which may result from them.
What this means is that in effect, we have no impression of necessary connection which derives from impressions of things in the external world.

Hume the turns to consider whether we may get the ideas of cause and necessary connection from the operations of our own minds. In particular, Hume is interested in the claim that "...we are conscious of internal power; while we feel, that, by the simple command of our will, we can move the organs of our body, or direct the faculties of our mind." This idea of power is an idea of reflection, since it comes from reflecting on the powers of our own minds. Hume proposes to examine this claim.

He begins by pointing out that though the influence of volition over the organs of the body is plainly a fact, it is equally plain that this fact is known only by experience "and can never be foreseen by any apparent energy or power in the cause which connects it with the effect and renders the one the infallible consequence of the other." So we are conscious that the motions of our bodies follows upon the command of our will, but "the means, by which this is effected...of this we are so far from being immediately conscious, that it must forever escape our most diligent inquiry." Hume goes on to give a variety of reasons why this is so. To know a power, Hume tells us, "we know the very circumstances in the cause, by which it is enabled to produce the effect. For these are supposed to by synonymous. We must, therefore know the cause and the effect and the relation between them." But we simply don't know the human soul in this way. We have no idea what this power of the will is to move our bodily organs or to direct our thought, we only know that there is such a power. Second, the command of the mind over itself and the body are limited and it is only by experience that we come to know these limits. We cannot "assign the ultimate reason of these boundaries, or show why the power is deficient in one case and not in another." Hume goes on to argue that experience tells us that causes and effects are conjoined with one another, but that we cannot conceive how they are connected. This claim that causes and effects are conjoined is an important piece of Hume's positive account of causality, just as his claim that we have no basis for saying that causes and effects are connected is an important part of his critique of the rationalist account of causality.

Hume ends this first section on Necessary Connection with a long reflection of the difficulties of the Occasionalism of Nicholas Malebranche and his followers. According to Malebrache God is the only cause. There is some interest in this because it is plain that Hume learned a thing or two about causality from Malebranche. Malebranche's critique of what we know about causes from the senses and the mind surely influenced Hume's critique of causality. But Malebranche has a positive account of causality which Hume also wants to reject. In effect, he argues that we have no more experience of divine causality of minds and bodies than we do of minds causing bodies to move, or one body causing another to move.

In Part II of the section on Necessary Connection Hume comes to a conclusion from the application of his microscope to the idea of necessary connection. He writes:

All events seem entirely loose and separate. One event follows another; but we never can observe any tie between them. They seem conjoined but never connected. And as we can have no idea of anything, which never appears to our outward sense or inward sentiment, the necessary conclusion seems to be, that we have no idea of connexion or power at all, and that these words are absolutely without any meaning, when employed either in philosophical reasoning, or in private life.
Wow! What a conclusion! The ideas of 'power' and 'necessary connection' are "absolutely without any meaning." This is pretty strong stuff. But why does Hume say that this seems to be the necessary conclusion? The answer is that we still have to give an account of why we are using such terms in the wrong ways in which we do.

Hume's answer is that we get the idea of 'necessary connection' from the constant conjunction of causes and effects. He writes that from a single instance of one event being followed by another, we are not entitled to form a general rule or to predict what will happen in similar cases, but "when one particular species of events has always, in all instances, been conjoined with one another, we make no scruple any longer in foretelling one upon the appearance of the other, and of employing that reasoning, which alone can assure us of any matter of fact or existence. We then call the one object Cause; the other, Effect" So, do "constant conjunction" and "necessary connection" mean the same thing? The answer I take it is that they ought to but that they do not. Hume says that "We suppose that there is some connexion between them; some power in the one, by which it infallibly produces the other, and operates with the greatest certainty and strongest necessity." -- but there is no legitimate basis in our experience for this additional claim. Instead, that claim is derived simply from the habit of the mind "upon the appearance of one event, to expect its usual attendant, and to believe that it will exist. This connexion therefore, which we feel in the mind, this customary transition of the imagination from one object to its usual attendant, is the sentiment or impression, from which we form the idea of power or necessary connexion." So, any claim that "connection" means anything stronger than "constant conjunction" is not justified.

We can now see some of the negative conclusions which follows from this analysis. I return to the list of claims about causal propositions given earlier:

  1. The claim that propositions about causation are necessary truths.
  2. Propositions about particular causes can be known a priori or prior to experience.
  3. Propositions about particular causes involve logical as well as causal connections.
  4. Because of the nature of causality, rationalist metaphysics of the kind practiced by Spinoza is possible.
The claim that propositions about causation are necessary truths, Hume rejects as going beyond what we can discover from experience. We cannot know propositions about particular causes prior to experience. The first time we experience a conjunction of a cause and and effect is just that, our first experience of such things. We cannot look at the cause a priori and deduce what the effect will be. Since we know only the conjunction and not the necessary connection between cause and effect, causality is purely a matter of experience and not a logical relation.