are (in some sense) material, we are led to the conclusion that all the actual qualities in percepts
are different from those in their unperceived causes, but that there is a certain structural similarity
between the system of percepts and the system of their causes. There is, for example, a correlation
between colours (as perceived) and wavelengths (as inferred by physicists). Similarly there must
be a correlation between space as an ingredient in percepts and space as an ingredient in the
system of unperceived causes of percepts. All this rests upon the maxim "same cause, same
effect," with its obverse, "different effects, different causes." Thus, e.g., when a visual percept A
appears to the left of a visual percept B, we shall suppose that there is some corresponding
relation between the cause of A and the cause of B.
We have, on this view, two spaces, one subjective and one objective, one known in experience
and the other merely inferred. But there is no difference in this respect between space and other
aspects of perception, such as colours and sounds. All alike, in their subjective forms, are known
empirically; all alike, in their objective forms, are inferred by means of a maxim as to causation.
There is no reason whatever for regarding our knowledge of space as in any way different from
our knowledge of colour and sound and smell.
With regard to time, the matter is different, since, if we adhere to the belief in unperceived causes
of percepts, the objective time must be identical with the subjective time. If not, we get into the
difficulties already considered in connection with lightning and thunder. Or take such a case as the
following: You hear a man speak, you answer him, and he hears you. His speaking, and his
hearing of your reply, are both, so far as you are concerned, in the unperceived world; and in that
world the former precedes the latter. Moreover his speaking precedes your hearing in the objective
world of physics; your hearing precedes your reply in the subjective world of percepts; and your
reply precedes his hearing in the objective world of physics. It is clear that the relation "precedes"
must be the same in all these propositions. While, therefore, there is an important sense in which
perceptual space is subjective, there is no sense in which perceptual time is subjective.
The above arguments assume, as Kant does, that percepts are caused by "things in themselves," or,
as we should say, by events in the world