made at
the same instant, or succeeding each other so quickly, that the
second takes place before the first has vanished.

As an instance of the first, we may mention musical chords, or the
sounds produced by the union of two or more tones at the same time.
We have another instance likewise in odours or smells; if two or more
perfumes be mixed together, a compound odour will be perceived,
different from any of them.

As an instance of the latter, if a paper painted of various colours
be made to revolve rapidly in a circle, a compound colour, different
from any of them, will be perceived. These observations apply
particularly to the senses we have mentioned, and likewise to taste:
but the sensations afforded us by touch do not seem capable of being
compounded in this manner.

There are many things necessary to perfect sensation, besides those
that have been mentioned. The degree and perfection of sensation will
depend much on the mind, and will be continually altered by delirium,
torpor, sleep, and other circumstances; much likewise depends on the
state of the organs with respect to preceding impressions; for if any
organ of sense have been subjected to a strong impression, it will
become nearly insensible of those which are weaker.

Of this innumerable instances may be given: an eye which has been
subjected to a strong light, becomes insensible of a weaker: and on
the contrary, if the organs of sense have been deprived of their
accustomed impressions for some time, they are affected by very
slight ones. Hence it is, that when a person goes from daylight into
a darkened room, he can at first see nothing; by degrees however he
begins to have an imperfect perception of the different objects, and
if he remain long enough, he will see them with tolerable
distinctness, though the quantity of light be the same as when he
entered the room, when they were invisible to him.

Sensation often arises from internal causes, without any external
impulse. To this source may be referred consciousness, memory,
imagination, volition, and other affections of the mind. These are
called the internal senses. The senses, whether internal or external,
have never been accurately reduced to classes, orders, or genera; the
external indeed are generally referred to five orders; namely,
seeing, hearing, smelling, tasting, and feeling, or touch. With
respect to the four first, the few qualities of external bodies which
each perceives may be eas