Helmholtz relates that, owing to this tendency, he has occasionally caught himself, on a dark night, entertaining the illusion that the comparatively bright objects visible in twilight were self-luminous.[18] Again, there are limits to the conscious separation of sensations which are received together, and this fact gives rise to illusion.

In general, the number of distinguishable sensations answers to the number of external causes; but this is not always the case, and here we naturally fall into the error of mistaking the number of the stimuli.

Reference has already been made to this fact in connection with the question whether consciousness can be mistaken as to the character of a present feeling.
The case of confusing two impressions when the sensory fibres involved are very near one another, has already been alluded to.

Both in touch and in sight we always take two or more points for one when they are only separated by an interval that falls below the limits of local discrimination.

It seems to follow from this that our perception of the world as a continuum, made up of points perfectly continuous one with another may, for what we know, be illusory.