al sense. The perception of external objects is termed
Sensation, that of internal phenomena (of the states of the mind itself)
Reflection. External and internal perception are the only windows
through which the light of ideas penetrates into the dark chamber of the
understanding. The two are not opened simultaneously, however, but one
after the other; since the perceptions of the sensible qualities of bodies,
unlike that of the operations of the mind itself, do not require an effort
of attention, they are the earlier. The child receives ideas of sensation
before those of reflection; internal perception presupposes external
perception.

In this distinction between sensation and reflection, we may recognize
an after-effect of the Cartesian dualism between matter and spirit.
The antithesis of substances has become a duality in the faculties of
perception. But while Descartes had so far forth ascribed precedence to the
mind in that he held the self-certitude of the ego to be the highest and
clearest of all truths and the soul to be better known than the body, in
Locke the relation of the two was reversed, since he made the perception
of self dependent on the precedent perception of external objects. This
antithesis was made still sharper in later thinking, when Condillac made
full use of the priority of sensation, which in Locke had remained without
much effect; while Berkeley, on the other hand, reduced external perception
to internal perception.

All original ideas are representations either of the external senses or
of the internal sense, or of both. And since, in the case of ideas of
sensation, there is a distinction between those which are perceived by a
single one of the external senses and those which come from more than one,
four classes of simple ideas result: (1) Those which come from one external
sense, as colors, sounds, tastes, odors, heat, solidity, and the like.
(2) Those which come from more than one external sense (sight and touch),
as extension, figure, and motion. (3) Reflection on the operations of our
minds yields ideas of perception or thinking (with its various modes,
remembrance, judging, knowledge, faith, etc.), and of volition or willing.
(4) From both external and internal perception there come into the mind the
ideas of pleasure and pain, existence, power, unity, and succession. These
are approximately our original ideas, which are related to knowledge as
the letters to written discourse; as all