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_.
These feelings, or their contraries, are the foundation of everything that we call a relation.