In certain exceptional cases the coalescence does not take place, as when I look at a distant object and hold a pencil just before my eyes.[23] And in this case the organized tendency to take one visual impression for one object asserts its force, and I tend to fall into the illusion of seeing two separate pencils.

If I do not wholly lapse into the error, it is because my experience has made me vaguely aware that double images under these circumstances answer to one object, and that if there were really two pencils present I should have four visual impressions.
Once more, it is a law of sensory stimulation that an impression persists for an appreciable time after the cessation of the action of the stimulus.

This "after sensation" will clearly lead to illusion, in so far as we tend to think of the stimulus as still at work.

It forms, indeed, as will be seen by-and-by, the simplest and lowest stage of hallucination.

Sometimes this becomes the first stage of a palpable error.