other parts of
the science, without themselves being believed on any ground except
observation. I assume, that is to say, a trained observer, with an
analytic attention, knowing the sort of thing to look for, and the sort
of thing that will be important. What he observes is, at the stage of
science which he has reached, a datum for his science. It is just as
sophisticated and elaborate as the theories which he bases upon it,
since only trained habits and much practice enable a man to make
the kind of observation that will be scientifically illuminating.
Nevertheless, when once it has been observed, belief in it is not based
on inference and reasoning, but merely upon its having been seen. In
this way its logical status differs from that of the theories which are
proved by its means.

In any science other than psychology the datum is primarily a
perception, in which only the sensational core is ultimately and
theoretically a datum, though some such accretions as turn the sensation
into a perception are practically unavoidable. But if we postulate an
ideal observer, he will be able to isolate the sensation, and treat this
alone as datum. There is, therefore, an important sense in which we
may say that, if we analyse as much as we ought, our data, outside
psychology, consist of sensations, which include within themselves
certain spatial and temporal relations.

Applying this remark to physiology, we see that the nerves and brain
as physical objects are not truly data; they are to be replaced, in
the ideal structure of science, by the sensations through which the
physiologist is said to perceive them. The passage from these sensations
to nerves and brain as physical objects belongs really to the initial
stage in the theory of physics, and ought to be placed in the reasoned
part, not in the part supposed to be observed. To say we see the
nerves is like saying we hear the nightingale; both are convenient but
inaccurate expressions. We hear a sound which we believe to be causally
connected with the nightingale, and we see a sight which we believe
to be causally connected with a nerve. But in each case it is only
the sensation that ought, in strictness, to be called a datum. Now,
sensations are certainly among the data of psychology. Therefore all the
data of the physical sciences are also psychological data. It remains
to inquire whether all the data of psychology are also data of physical
science, and especially of physiol