Now in so far as the above considerations depend upon supposing that
there are other people besides ourselves, they beg the very question
at issue. Other people are represented to me by certain sense-data,
such as the sight of them or the sound of their voices, and if I had
no reason to believe that there were physical objects independent of
my sense-data, I should have no reason to believe that other people
exist except as part of my dream. Thus, when we are trying to show
that there must be objects independent of our own sense-data, we
cannot appeal to the testimony of other people, since this testimony
itself consists of sense-data, and does not reveal other people's
experiences unless our own sense-data are signs of things existing
independently of us. We must therefore, if possible, find, in our own
purely private experiences, characteristics which show, or tend to
show, that there are in the world things other than ourselves and our
private experiences.

In one sense it must be admitted that we can never prove the existence
of things other than ourselves and our experiences. No logical
absurdity results from the hypothesis that the world consists of
myself and my thoughts and feelings and sensations, and that
everything else is mere fancy. In dreams a very complicated world may
seem to be present, and yet on waking we find it was a delusion; that
is to say, we find that the sense-data in the dream do not appear to
have corresponded with such physical objects as we should naturally
infer from our sense-data. (It is true that, when the physical world
is assumed, it is possible to find physical causes for the sense-data
in dreams: a door banging, for instance, may cause us to dream of a
naval engagement. But although, in this case, there is a physical
cause for the sense-data, there is not a physical object corresponding