particular with other appearances
in the same piece of matter, rather than with other appearances in the
same perspective. That is to say, they group together particulars having
the same "active" place, while psychology groups together those having
the same "passive" place. Some particulars, such as images, have no
"active" place, and therefore belong exclusively to psychology.

We can now understand the distinction between physics and psychology.
The nerves and brain are matter: our visual sensations when we look
at them may be, and I think are, members of the system constituting
irregular appearances of this matter, but are not the whole of the
system. Psychology is concerned, inter alia, with our sensations when we
see a piece of matter, as opposed to the matter which we see. Assuming,
as we must, that our sensations have physical causes, their causal laws
are nevertheless radically different from the laws of physics, since
the consideration of a single sensation requires the breaking up of
the group of which it is a member. When a sensation is used to verify
physics, it is used merely as a sign of a certain material phenomenon,
i.e. of a group of particulars of which it is a member. But when it is
studied by psychology, it is taken away from that group and put
into quite a different context, where it causes images or voluntary
movements. It is primarily this different grouping that is
characteristic of psychology as opposed to all the physical sciences,
including physiology; a secondary difference is that images, which
belong to psychology, are not easily to be included among the aspects
which constitute a physical thing or piece of matter.

There remains, however, an important question, namely: Are mental events
causally dependent upon physical events in a sense in which the converse
dependence does not hold? Before we can discuss the answer to this
question, we must first be clear as to what our question means.

When, given A, it is possible to infer B, but given B, it is not
possible to infer A, we say that B is dependent upon A in a sense in
which A is not dependent upon B. Stated in logical terms, this amounts
to saying that, when we know a many-one relation of A to B, B is
dependent upon A in respect of this relation. If the relation is a
causal law, we say that B is causally dependent upon A. The illustration
that chiefly concerns us is the system of appearances of a physical
object. We can, broadly speakin