re laden with toys and no box is there, the word-habit arises and
he calls 'box.'" This is inadequate as it stands, since the habit has
been to use the word when the box is present, and we have to explain its
extension to cases in which the box is absent.

Having admitted images, we may say that the word "box," in the absence
of the box, is caused by an image of the box. This may or may not
be true--in fact, it is true in some cases but not in others. Even,
however, if it were true in all cases, it would only slightly shift our
problem: we should now have to ask what causes an image of the box to
arise. We might be inclined to say that desire for the box is the cause.
But when this view is investigated, it is found that it compels us to
suppose that the box can be desired without the child's having either an
image of the box or the word "box." This will require a theory of desire
which may be, and I think is, in the main true, but which removes desire
from among things that actually occur, and makes it merely a convenient
fiction, like force in mechanics.* With such a view, desire is no longer
a true cause, but merely a short way of describing certain processes.

* See Lecture III, above.

In order to explain the occurrence of either the word or the image
in the absence of the box, we have to assume that there is something,
either in the environment or in our own sensations, which has frequently
occurred at about the same time as the word "box." One of the laws which
distinguish psychology (or nerve-physiology?) from physics is the
law that, when two things have frequently existed in close temporal
contiguity, either comes in time to cause the other.* This is the basis
both of habit and of association. Thus, in our case, the arms full of
toys have frequently been followed quickly by the box, and the box in
turn by the word "box." The box itself is subject to physical laws, and
does not tend to be caused by the arms full of toys, however often it
may in the past have followed them--always provided that, in the case in
question, its physical position is such that voluntary movements cannot
lead to it. But the word "box" and the image of the box are subject to
the law of habit; hence it is possible for either to be caused by the
arms full of toys. And we may lay it down generally that, whenever we
use a word, either aloud or in inner speech, there is some sensation
or image (either of which may be itself a word) whic