Note 67 — APPENDIX, NO. 2.
Reading edition
A Sensation is an impression made by some external object on the Senses; the motion thus excited is propagated along the appropriate nerve, until it reaches the Sensory in the Brain, and it is there and there only, felt or perceived.
An Idea, is a motion in the Brain, excited there either by the laws of association to which that organ is subject, or by some accidental state of the system in general, or that organ in particular, without the intervention of an impression on the Senses ab extra as the cause of it. Such a motion being similar to a sensation formerly excited, and being also felt or perceived is the correspondent Idea.