The Scottish philosopher David Hume (1711-1776) argued that concepts must be rooted in sense experiences in order for them to be meaningful. In section two of his famous publication entitled An Enquiry Concerning Human Understanding, he warned that “when we entertain … any suspicion that a philosophical term is employed without any meaning or idea (as is but too frequent), we need but enquire from what impression is that supposed idea derived? And if it be impossible to assign any, this will serve to confirm our suspicion” that the term is meaningless.
Hume’s criterion of meaning was the starting point for his investigations into the concept of causation and whether the concept had any basis in sense experience. Hume thought that the idea of causation as a necessary connection between objects was problematic because when we look at our experience carefully, we find no such connection. Some objects may appear to be connected to others by necessity, but what we actually observe is a contingent relation of succession, whereby one kind of object (A) is regularly followed by another kind of object (B). For example, there may appear to be a necessary connection between a moving brick (A) and some shattered glass (B), but what we actually see is the moving brick and the shattered glass in rapid succession. So why do we infer a necessary link between the two?
Hume’s answer is that we infer it out of custom. Custom is the mind’s tendency to expect one kind of thing to follow another kind of thing because this has happened frequently in the past. Over time, the mind starts to believe that there is something necessary about their co-occurrence, but this belief goes beyond what we actually observe. We tend to believe there is necessary connection between a moving brick and shattering glass because we expect this on the basis of past experiences. But an expectation based on custom is not the same as a necessary connection. Thus, if Hume is correct that we observe no necessary relations between objects, it follows that the concept of causation (so defined) has not basis in sense experience and should therefore be rejected as meaningless.
This conclusion motivated Hume to provide an alternative definition of causation based on what we actually observe – namely the relation of regular succession between kinds of objects. In other words, “we may define a cause to be an object, followed by another, and where all the objects similar to the first are followed by objects similar to the second” (section 7). To say that A causes B just is to say that objects of kind A are regularly followed by objects of kind B; or to use our example, to say that the moving brick caused the glass to shatter just is to say that moving bricks and shattering glass are related in experience by regular succession. By reducing causation to succession, Hume could argue that the concept of causation was meaningful after all.
One problem with Hume’s argument from sense experience is a problem we all face when trying to detect whether we are observing something directly or simply making judgments about it – or both! One person will say we directly observe the relation of causation; others will say no, we infer it by habit. It is hard to know how to resolve such disputes, especially in cases where inanimate objects are involved. Even David Lewis (2004), a long time admirer of Hume and defender of his own brand of reductionism about causation, admits that it is
…famously difficult to draw the line between what’s true according to perceptual experience all by itself and what’s true according to a system of beliefs shaped partly by perceptual experience and partly by previous beliefs…. Do I see that the one thing causes the other? Or do I infer it from what I do see, together with my background knowledge about the ways of the world? I don’t know, and I don’t know how to find out.
I sympathize with Lewis’ hesitation about Hume, but I would argue that the “line” he refers to is clearer in cases of human action, especially when we notice ourselves trying to act. When we try – when we push or pull objects around us, we immediately experience ourselves exercising power (or “oomph!”). We may succeed or fail in our trying – that is, objects can give way or resist our efforts – but we nevertheless experience our own powers when we do this. Indeed, some philosophers take this as evidence that when we try, we directly observe causation as it is happening. For example, Richard Swinburne argues that we discover causation at the very least (and perhaps primarily) by noticing our basic tryings, not by observing regular successions between objects. If this is correct, then Hume’s objection to the observability of causation fails.
As it happens, Hume believed that actions were no exception to his view. Why? Because we only anticipate a connection between our acts of trying (on the one hand) and the effects of our trying (on the other) because we regularly observe them in succession. As he writes in the appendix of his Treatise of Human Nature, “[the will] has no more a discoverable connection with its effects, than any material cause has with its proper effect”; such a connection “could not be foreseen without the experience of their constant conjunction.” In other words, we infer a connection between an act of trying and its effect because we are in the habit of anticipating one from the other. The experience of causal power is a matter of anticipating these past successions in our minds, nothing more.
What do you (the reader) think about Hume’s analysis of causation? Has he provided a good response to Swinburne’s rebuttal?