he patient were
placed in such conditions as this, Diderot thinks there would be more
profit in questioning a blind person of good sense, than in the answers
of an uneducated person receiving sight for the first time under
abnormal and bewildering circumstances.[63] In this he was undoubtedly
right. If the experiment could be prepared under the delicate conditions
proper to make it demonstrative evidence, it would be final. But the
experiment had certainly not been so prepared in his time, and probably
never will be.[64]

Read in the light of the rich and elaborate speculative literature which
England is producing in our own day, Diderot's once famous Letter on the
Blind seems both crude and loose in its thinking. Yet considering the
state of philosophy in France at the time of its appearance, we are
struck by the acuteness, the good sense, and the originality of many of
its positions. It was the first effective introduction into France of
these great and fundamental principles; that all knowledge is relative
to our intelligence, that thought is not the measure of existence, nor
the conceivableness of a proposition the test of its truth, and that our
experience is not the limit to the possibilities of things. That is an
impatient criticism which dismisses the French philosophers with some
light word as radically shallow and impotent. Diderot grasped the
doctrine of Relativity in some of the most important and far-reaching of
all its bearings. The fact that he and his allies used the doctrine as a
weapon of combat against the standing organisation, is exactly what
makes their history worth writing about. The standing organisation was
the antagonistic doctrine incarnate. It made anthropomorphism and the
absolute the very base and spring alike of individual and of social
life. No growth was possible until this speculative base had been
transformed. Hence the profound significance of what looks like a mere
discussion of one of the minor problems of metaphysics. Diderot was not
the first to discover Relativity, nor did he establish it; but it was he
who introduced it into the literature of his country at the moment when
circumstances were ripe for it.

Condillac, as we have said, had published his first work, the Essay on
the Origin of Human Knowledge, three years before (1746). This was a
simple and undeveloped rendering of the doctrine of Locke, that the
ultimate source of our notions lies in impressions made upon the sen