what can be learnt by external observation. I am very anxious that no
ideas should be attached to the words "purpose" and "desire" beyond
those involved in the above definitions.

We have not so far considered what is the nature of the initial stimulus
to a behaviour-cycle. Yet it is here that the usual view of desire seems
on the strongest ground. The hungry animal goes on making movements
until it gets food; it seems natural, therefore, to suppose that the
idea of food is present throughout the process, and that the thought of
the end to be achieved sets the whole process in motion. Such a view,
however, is obviously untenable in many cases, especially where instinct
is concerned. Take, for example, reproduction and the rearing of the
young. Birds mate, build a nest, lay eggs in it, sit on the eggs, feed
the young birds, and care for them until they are fully grown. It
is totally impossible to suppose that this series of actions, which
constitutes one behaviour-cycle, is inspired by any prevision of the
end, at any rate the first time it is performed.* We must suppose that
the stimulus to the performance of each act is an impulsion from behind,
not an attraction from the future. The bird does what it does, at each
stage, because it has an impulse to that particular action, not because
it perceives that the whole cycle of actions will contribute to the
preservation of the species. The same considerations apply to other
instincts. A hungry animal feels restless, and is led by instinctive
impulses to perform the movements which give it nourishment; but the act
of seeking food is not sufficient evidence from which to conclude that
the animal has the thought of food in its "mind."

* For evidence as to birds' nests, cf. Semon, "Die Mneme,"
pp. 209, 210.

Coming now to human beings, and to what we know about our own actions,
it seems clear that what, with us, sets a behaviour-cycle in motion is
some sensation of the sort which we call disagreeable. Take the case
of hunger: we have first an uncomfortable feeling inside, producing a
disinclination to sit still, a sensitiveness to savoury smells, and an
attraction towards any food that there may be in our neighbourhood. At
any moment during this process we may become aware that we are hungry,
in the sense of saying to ourselves, "I am hungry"; but we may have been
acting with reference to food for some time before this moment. While we
are talking or reading, we ma