must be bodily movements in whose causation the special properties
of nervous tissue are involved. The cycle ends in a condition of
quiescence, or of such action as tends only to preserve the status quo.
The state of affairs in which this condition of quiescence is achieved
is called the "purpose" of the cycle, and the initial mental occurrence
involving discomfort is called a "desire" for the state of affairs that
brings quiescence. A desire is called "conscious" when it is accompanied
by a true belief as to the state of affairs that will bring quiescence;
otherwise it is called "unconscious." All primitive desire is
unconscious, and in human beings beliefs as to the purposes of desires
are often mistaken. These mistaken beliefs generate secondary desires,
which cause various interesting complications in the psychology of human
desire, without fundamentally altering the character which it shares
with animal desire.

LECTURE IV. INFLUENCE OF PAST HISTORY ON PRESENT OCCURRENCES IN LIVING
ORGANISMS

In this lecture we shall be concerned with a very general characteristic
which broadly, though not absolutely, distinguishes the behaviour
of living organisms from that of dead matter. The characteristic in
question is this:

The response of an organism to a given stimulus is very often dependent
upon the past history of the organism, and not merely upon the stimulus
and the HITHERTO DISCOVERABLE present state of the organism.

This characteristic is embodied in the saying "a burnt child fears the
fire." The burn may have left no visible traces, yet it modifies the
reaction of the child in the presence of fire. It is customary to assume
that, in such cases, the past operates by modifying the structure of the
brain, not directly. I have no wish to suggest that this hypothesis is
false; I wish only to point out that it is a hypothesis. At the end of
the present lecture I shall examine the grounds in its favour. If we
confine ourselves to facts which have been actually observed, we must
say that past occurrences, in addition to the present stimulus and
the present ascertainable condition of the organism, enter into the
causation of the response.

The characteristic is not wholly confined to living organisms. For
example, magnetized steel looks just like steel which has not been
magnetized, but its behaviour is in some ways different. In the case
of dead matter, however, such phenomena are less frequent and important
t