Because a man, by his senses and every day’s observation, may easily satisfy himself what the simple ideas are which their several names that are in common use stand for; they being but few in number, and such as, if he doubts or mistakes in, he may easily rectify by the objects they are to be found in.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“First, then, I say, that when the truth of our ideas is judged of by the conformity they have to the ideas which other men have, and commonly signify by the same name, they may be any of them false. But yet SIMPLE IDEAS are least of all liable to be so mistaken. Because a man, by his senses and every day’s observation, may easily satisfy himself what the simple ideas are which their several names that are in common use stand for; they being but few in number, and such as, if he doubts or mistakes in, he may easily rectify by the objects they are to be found in. Therefore it is seldom that any one mistakes in his names of simple ideas, or applies the name red to the idea green, or the name sweet to the idea bitter: much less are men apt to confound the names of ideas belonging to different senses, and call a colour by the name of a taste, &c.” source