ng a firebrand in a circular
manner, by which the impression of a circle is caused, instead of a
moving point: and, with respect to hearing, it may be observed, that
when children run with a stick quickly along railing, or when a drum
is beaten quickly, the idea of a continued sound is produced, because
the impression remains some time: for it is evident, that the sounds
produced in succession are perfectly distinct and insulated.

Sensation likewise depends, in a great measure, on the state of the
mind, and on the degree of attention which it gives. For if we are
engaged in attention to any object, we are insensible of the
impressions made upon us by others, though they are sufficiently
strong to affect us at other times. Thus, when our attention is fixed
strongly upon any particular object, we become insensible of the
various noises that surround us, though these may be sometimes very
loud. On the contrary, if our attention be upon the watch, we can
perceive slight, and almost neglected impressions, while those of
greater magnitude become insensible. The ticking of a clock becomes
insensible to us from repetition, but if we attend to it, we become
easily sensible of it, though at the same time we become insensible
of much stronger impressions, such as the rattling of coaches in the
streets.

The attention depends in some degree on the will, but is generally
given to those impressions which are particularly strong, new,
pleasant, or disagreeable; in short, to those which particularly
affect the mind. Hence it is, that things which are new, produce the
most vivid impressions, which gradually grow fainter, and at last
become imperceptible.

There is one circumstance respecting sensation, which will probably
account for our only perceiving those impressions to which the mind
attends: and this is, that the mind is incapable of perceiving more
than one impression at a time: the more accurately we examine this,
the greater reason we shall have to think it true; but the mind can
turn its attention so quickly, from one object to another, that at
first sight, we are led to believe, that we are able to attend to
several at the same time.

But though the mind cannot perceive or attend to various sensations
at the same time, yet if two or more of these are capable of uniting
in such a manner as to produce a compound sensation, this may be
perceived by the mind.

This compound sensation may be produced either by impressions