e second question, tentatively and for
the present, I should answer in favour of introspection; I think that
images, in the actual condition of science, cannot be brought under the
causal laws of physics, though perhaps ultimately they may be. The
third question I should answer adversely to introspection I think that
observation shows us nothing that is not composed of sensations and
images, and that images differ from sensations in their causal laws, not
intrinsically. I shall deal with the three questions successively.

(1) PUBLICITY OR PRIVACY OF WHAT IS OBSERVED. Confining ourselves, for
the moment, to sensations, we find that there are different degrees
of publicity attaching to different sorts of sensations. If you feel a
toothache when the other people in the room do not, you are in no way
surprised; but if you hear a clap of thunder when they do not, you begin
to be alarmed as to your mental condition. Sight and hearing are the
most public of the senses; smell only a trifle less so; touch, again, a
trifle less, since two people can only touch the same spot successively,
not simultaneously. Taste has a sort of semi-publicity, since people
seem to experience similar taste-sensations when they eat similar foods;
but the publicity is incomplete, since two people cannot eat actually
the same piece of food.

But when we pass on to bodily sensations--headache, toothache, hunger,
thirst, the feeling of fatigue, and so on--we get quite away from
publicity, into a region where other people can tell us what they feel,
but we cannot directly observe their feeling. As a natural result of
this state of affairs, it has come to be thought that the public senses
give us knowledge of the outer world, while the private senses only give
us knowledge as to our own bodies. As regards privacy, all images, of
whatever sort, belong with the sensations which only give knowledge of
our own bodies, i.e. each is only observable by one observer. This is
the reason why images of sight and hearing are more obviously different
from sensations of sight and hearing than images of bodily sensations
are from bodily sensations; and that is why the argument in favour of
images is more conclusive in such cases as sight and hearing than in
such cases as inner speech.

The whole distinction of privacy and publicity, however, so long as we
confine ourselves to sensations, is one of degree, not of kind. No
two people, there is good empirical reason to t