What is it that is supposed to be so unobservable about causation? This question has nagged at me since my days as an undergraduate (and there has recently been a lot of discussion of it over at Brains, which inspired me to post on it). It had always seemed to me that the causal relation was entirely observable. We even have evidence that we have been observing it since before we could talk. For instance from the cradle we witness the passing of the sun behind clouds and the subsequent darkness, we witness objects falling to the ground, we witness that motion is attended with sound, and etc. Nor are these examples special; cases like these can be multiplied indefinitely. We can see water extinguish flame. While walking about I kick a stone and see my foot cause the rock to begin its trajectory. I can see the acid turn the litmus paper red. Pointing a magnifying glass at a piece of paper on a sunny day will cause the paper to smoke and turn brown, eventually catching fire. A small dog biting at my ankles will cause a pinching sensation and perhaps annoyance! I take these to be examples of seeing A cause B, seeing A causing B and feeling A cause B.
So why think that the causal relation is unobservable? I take the canonical text on this to be Section seven of Hume’s Enquiry. For instance in paragraph six of that section he says,
When we look about us towards external objects, and consider the operation of causes, we are never able, in any single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find, that the one ball does, actually, in fact, follow the other. The impulse of one billiard-ball is attended with motion in the second. This is the whole as it 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, any thing which can suggest the idea of power or necessary connexion. (Hume 1999)
No doubt it was passages like this that lead to Kant’s rather rude awakening from his dogmatic slumber. It seems to me to have plunged us into one. In the above passage, which is completely typical of that section, Hume seems to be saying that since we cannot see that the connection between these events is necessary, which is to say that we cannot see why what happens is the ‘infallible consequence’ of the first event as opposed to some other event, there is no way that the idea of causation could be taken from anything perceptible in the outside world. It follows from what he says that the causal relation is not observable; something that makes no impression on the human senses would have to be. The intuition that we have to see the necessity of the causal relation in order to see it at all has guided discussions of the observability of the causal relation. It seems to me that Hume here fails to make a distinction between what we observe and how we analyze what we see. What we observe is a relation; it is when we turn to how to characterize that relation that necessity is involved.
As C.J. Ducasse pointed out, Hume’s claim about the unobservability of causation would be true only under “the assumption that a ‘connection’ is an entity of the same sort as the terms themselves between which it holds…” (Ducasse 1993, p 131). Hume’s mistake was to look for the sense impression of a relation. This we will not find unless relations exist in the same way as the things they relate. But the relation between two objects is not itself a third object! Consider two objects, one to the left of the other. We do not see the relation ‘to the left of,’ but rather see one thing as being to the left of another. It does not even make sense to ask ‘what does the relation ‘to the left of’ look like?’ If asked this question all we could do is describe the position of the objects along with the definition of ‘left,’ and ‘right.’
Any further questioning about what ‘left of’ looks like would betray a category mistake. That is, mistaking a relation for an entity of the sort that it holds between. By way of comparison imagine Hume arguing that ‘left of’ is unobservable. I mean all we see is object A and Object B no where is there an impression of ‘left of,’ and so strictly speaking our idea of ‘left of’ is meaningless. But this is ridiculous! The argument seems to work against any relation and so proves too much.
When we see one ball hit the other we observe that the two balls are related, one causes the other to move, but we do not see that the relation is necessary even though it is in fact a necessary one. Just as we can see X moving without seeing that X is going 22 MPH, or we can see X being 3 ft away from Y without seeing that X is 3 ft away from Y. Despite the fact that I do not see that X is three feet from me I do see the relation ‘3 ft away from,’ I just don’t know that it is this relation. We see relation X qua relation while not seeing that X is such and such a relation. So on the view I am advocating we see A and we see B and we see A cause B but whether or not we see THAT the relation is necessary is a different question.