242 An Essay on the Signs of Ideas. 
a word which is used, both in conversation and 
in writing, with more vagueness, and with a 
greater variety of meanings, than almostany 
other.* 
Every individual of the human species, is, 
at various times, conscious of various feelings, 
arising apparently from different causes. These 
feelings may be distributed into four prin- 
cipal classes, and designated as follows: 1. 
Simple and original sensations; 2. Remem- 
bered sensations, or Ideas; 3. Sensations of 
emotion; and 4. The Feeling of volition. 
Simple and original sensations are those 
feelings which are occasioned,—by the appli- 
cation of external agents to the organs of sense, 
as light to the eye, aerial vibrations to the ear, 
odorous particles to the inner membrane of 
the nose, sapid bodies to the tongue and pa- 
late, heat, cold, and all sorts of resisting sur- 
* The great, and, in most respects, justly celebrated 
Locke, throughout the whole of his essay, employs the 
word Idea in the most obscure and ambiguous manner, 
The consequence is, that many rise from the study of his 
work, which is really the production of a surprising ge- 
nius, without finding that they have understood it. I have 
heard of one who, though he studied it for some years, 
complained that he never could ascertain what Locke meant 
by the word Idea. The fact is, that Locke made that term 
synonymous with the terms opinion, notion, principle, 
knowledge, theory, hypothesis, desire, instinct, habit. 
