He accepted, therefore, from common sense the belief that physical
reality is composed of a multitude of separate existences that act on
one another, and tried to conceive mental life strictly on the same
analogy. His theory of experience, therefore, closely parallels the
atomistic theory of matter. Just as the physicist explains bodies as
collections of discrete particles, so Hume reduced all the contents of
the mind to a number of elementary sensations. Whether the mind was
reflecting on its own internal ideas, or whether it was undergoing
impressions which it supposed to come from an external source, all that
was really happening was a succession of detached sensations. It seemed
to Hume indisputable that every distinct perception (or 'impression')
was a distinct existence, and that all 'ideas' were equally distinct,
though fainter, copies of impressions. Beyond impressions and ideas it
was unnecessary to look. Thus to look at a chessboard was to have a
number of sensations of black and white arranged in a certain order, to
listen to a piece of music was to experience a succession of loud and
soft auditory sensations, to handle a stone was to receive a group of
sensations of touch. To suppose that anything beyond these sensory units
was ever really experienced was futile fiction. Experience was a mosaic,
of which the stones were the detached sensations, and their washed-out
copies, the ideas.

If this analysis of the mind were correct--and its correctness was not
disputed for more than a hundred years, for were not the sensations
admitted to be the ultimate analysis of all that was perceived?--the
common-sense belief that knowledge revealed a world outside the thinker
was, of course, erroneous. For common sense could hardly treat 'things'
as merely 'sensations' artificially grouped together in space, each
'thing' being a complex of a number of sensations having relation to
similar complexes. It held rather that the successive appearances of