sensations by their
correlations, not by what they are in themselves. It is now necessary,
however, to examine the notion of a "datum," and to obtain, if possible,
a definition of this notion.

The notion of "data" is familiar throughout science, and is usually
treated by men of science as though it were perfectly clear.
Psychologists, on the other hand, find great difficulty in the
conception. "Data" are naturally defined in terms of theory of
knowledge: they are those propositions of which the truth is known
without demonstration, so that they may be used as premisses in proving
other propositions. Further, when a proposition which is a datum asserts
the existence of something, we say that the something is a datum, as
well as the proposition asserting its existence. Thus those objects
of whose existence we become certain through perception are said to be
data.

There is some difficulty in connecting this epistemological definition
of "data" with our psychological analysis of knowledge; but until such
a connection has been effected, we have no right to use the conception
"data."

It is clear, in the first place, that there can be no datum apart from a
belief. A sensation which merely comes and goes is not a datum; it only
becomes a datum when it is remembered. Similarly, in perception, we do
not have a datum unless we have a JUDGMENT of perception. In the sense
in which objects (as opposed to propositions) are data, it would seem
natural to say that those objects of which we are conscious are data.
But consciousness, as we have seen, is a complex notion, involving
beliefs, as well as mnemic phenomena such as are required for perception
and memory. It follows that no datum is theoretically indubitable, since
no belief is infallible; it follows also that every datum has a greater
or less degree of vagueness, since there is always some vagueness in
memory and the meaning of images.

Data are not those things of which our consciousness is earliest in
time. At every period of life, after we have become capable of thought,
some of our beliefs are obtained by inference, while others are not.
A belief may pass from either of these classes into the other, and may
therefore become, or cease to be, a belief giving a datum. When, in what
follows, I speak of data, I do not mean the things of which we feel sure
before scientific study begins, but the things which, when a science
is well advanced, appear as affording grounds for