presence of food. Two conditions must be met: the
internal state of hunger and the external stimulus of food; then, and
then only, will the eating reaction take place.

Hunger, though a tendency to eat, does not arouse the eating movements
while the stimulus of present food is lacking; but, for all that,
hunger does arouse immediate action. It typically arouses the
preparatory reactions of seeking food. Any such reaction is at the
same time a response to some actually present stimulus. Just as the
dog coming at your whistle was responding every instant of his
progress to some particular object--leaping fences, dodging trees--so
the dog aroused to action by the pangs of hunger begins at once to
respond to present objects. He does not start to eat them, because
they are not the sort of stimuli that produce this response, but he
responds by dodging them or finding his way by them in his quest for
food. The responses that the hungry dog makes to other objects than
{81} food are preparatory reactions, and these, if successful, put the
dog in the presence of food. That is to say, the _preparatory
reactions provide the stimulus that is necessary to arouse the
end-reaction_. They bring the individual to the stimulus, or the
stimulus to the individual.

[Illustration: Fig. 23.--A stimulus arouses the tendency towards the
end-reaction, R, but (as indicated by the dotted line), T is not by
itself sufficient to arouse R; but T can and does arouse P, a
preparatory reaction, and P (or some external result directly produced
by P), cooeperating with T, gives rise to R.]

What we can say about the modus operandi of hunger, then, amounts to
this: Hunger is an inner state and adjustment predisposing the
individual to make eating movements in response to the stimulus of
present food; in the absence of food, hunger predisposes to such other
responses to various stimuli as will bring the food stimulus into
play, and thus complete the conditions necessary for the eating
reaction. In general, _an aroused reaction-tendency predisposes the
individual to make a certain end-reaction when the proper stimulus for
that reaction is present; otherwise, it predisposes him to respond to
other stimuli, which are present, by preparatory reactions that
eventually bring to bear on the individual the stimulus required to
arouse the end-reaction_.

Let us apply our formula to one more simple case. While reading in the
late afternoon, I find the dayli