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 Homer is composed out of only twenty-four letters, so these few simple ideas constitute all the material of knowledge.  The mind can neither have more nor other simple ideas than those which are furnished to it by these two sources of experience.

Locke differs from Descartes again in regard to extension and thought.  Extension does not constitute the essence of matter, nor thought the essence of mind.  Extension and body are not the same; the former is presupposed by the latter as its necessary condition, but it is the former alone which yields mathematical matter.  The essence of physical matter consists rather in solidity:  where impenetrability is found there is body, and the converse; the two are absolutely inseparable.  With space the case is different.  I cannot conceive unextended matter, indeed, but I can easily conceive immaterial extension, an unfilled space Further, if the essence of the soul consisted in thought, it must be always thinking.  As the Cartesians maintained, it must have ideas as soon as it begins to be, which is manifestly contrary to experience.  Thinking is merely an activity of the mind, as motion