As an argument, this seems invalid. We believe in many relations of time and place which we
cannot perceive: we think that time extends backwards and forwards, and space beyond the walls
of our room. Hume's real argument is that, while we sometimes perceive relations of time and
place, we never perceive causal relations, which must therefore, if admitted, be inferred from
relations that can be perceived. The controversy is thus reduced to one of empirical fact: Do we, or
do we not, sometimes perceive a relation which can be called causal? Hume says no, his
adversaries say yes, and it is not easy to see how evidence can be produced by either side.
I think perhaps the strongest argument on Hume's side is to be derived from the character of
causal laws in physics. It appears that simple rules of the form "A causes B" are never to be
admitted in science, except as crude suggestions in early stages. The causal laws by which such
simple rules are replaced in well-developed sciences are so complex that no one can suppose them
given in perception; they are all, obviously, elaborate inferences from the observed course of
nature. I am leaving out of account modern quantum theory, which reinforces the above
conclusion. So far as the physical sciences are concerned, Hume is wholly in the right: such
propositions as "A causes B" are never to be accepted, and our inclination to accept them is to be
explained by the laws of habit and association. These laws themselves, in their accurate form, will
be elaborate statements as to nervous tissue--primarily its physiology, then its chemistry, and
ultimately its physics.
The opponent of Hume, however, even if he admits the whole of what has just been said about the
physical sciences, may not yet admit himself decisively defeated. He may say that in psychology
we have cases where a causal relation can be perceived. The whole conception of cause is
probably derived from volition, and it may be said that we can perceive a relation, between a
volition and the consequent act, which is something more than invariable sequence. The same
might be said of the relation between a sudden pain and a cry. Such views, however, are rendered
very difficult by physiology. Between the will to move my arm and the consequent movement
there is a long chain of causal intermediaries consisting of processes in the nerves and muscles.
We perceive only the end terms of this process, the volition and the movement, and if we think we
see a direct causal