"These", he writes, "are two very different things and carefully to
be distinguished: it being one thing to perceive and know the idea
of white or black, and quite another to examine what kind of
particles they must be, and how arranged ... to make any object
appear white or black." "A man infallibly knows, as soon as ever he
has them in his mind, that the ideas he calls white and round are
the very ideas they are, and that they are not other ideas which he
calls red or square.... This ... the mind ... always perceives at
first sight; and if ever there happen any doubt about it, it will
always be found to be about the names and not the ideas
themselves."

This sounds like high Platonic doctrine for a philosopher of the Left; but
Locke's utilitarian temper very soon reasserted itself in this subject.
Mathematical ideas were not only lucid but true: and he demanded this
truth, which he called "reality", of all ideas worthy of consideration:
mere ideas would be worthless. Very likely he forgot, in his philosophic
puritanism, that fiction and music might have an intrinsic charm. Where
the frontier of human wisdom should be drawn in this direction was clearly
indicated, in Locke's day, by Spinoza, who says:

"If, in keeping non-existent things present to the imagination, the
mind were at the same time aware that those things did not exist,
surely it would regard this gift of imagination as a virtue in its
own constitution, not as a vice: especially if such an imaginative
faculty depended on nothing except the mind's own nature: that is
to say, if this mental faculty of imagination were free".

But Locke had not so firm a hold on truth that he could afford to play
with fancy; and as he pushed forward the claims of human jurisdiction