When a red light flashes across the field of vision, there arises in the
mind an "impression of sensation"--which we call red. It appears to me
that this sensation, red, is a something which may exist altogether
independently of any other impression, or idea, as an individual
existence. It is perfectly conceivable that a sentient being should have
no sense but vision, and that he should have spent his existence in
absolute darkness, with the exception of one solitary flash of red
light. That momentary illumination would suffice to give him the
impression under consideration; and the whole content of his
consciousness might be that impression; and, if he were endowed with
memory, its idea.
Such being the state of affairs, suppose a second flash of red light to
follow the first. If there were no memory of the latter, the state of
the mind on the second occasion would simply be a repetition of that
which occurred before. There would be merely another impression.
But suppose memory to exist, and that an idea of the first impression is
generated; then, if the supposed sentient being were like ourselves,
there might arise in his mind two altogether new impressions. The one is
the feeling of the _succession_ of the two impressions, the other is the
feeling of their _similarity_.
Yet a third case is conceivable. Suppose two flashes of red light to
occur together, then a third feeling might arise which is neither
succession nor similarity, but that which we call _co-existence_.
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