For his investigation Hooke chose the point of a needle and a
knife-edge, as providing the best representatives among physical
objects of point and straight line. In the sketches here reproduced we
may see how Hooke made clear to his readers how little these two
things, when observed through the microscope, resemble what is seen by
the unaided eye. This fact convinced Hooke that the apparent agreement
between the world of perception and the world of ideas rests on nothing
more solid than an optical limitation (Plate I).

Compared with the more refined methods of present-day thought, Hooke's
procedure may strike us as somewhat primitive. Actually he did nothing
more than has since been done times without number; for the scientist
has become more and more willing to allow artificially evoked
sense-perceptions to dictate the thoughts he uses in forming a
scientific picture of the world.

In the present context we are concerned with the historical import of
Hooke's procedure. This lies in the fact that, immediately after
Descartes had satisfied himself that in thinking man had the one sure
guarantee of his own existence, Hooke proved in a seemingly indubitable
manner that thinking was entirely divorced from reality. It required
only another century for philosophy to draw from this the unavoidable
consequence. It appeared in the form of Hume's philosophic system, the
outcome of which was universal scepticism.

As we shall see in due course, Hume's mode of reasoning continues to
rule scientific thought even to-day, quite irrespective of the fact
that science itself claims to have its philosophical parent in Kant,
the very thinker who devoted his life's work to the refutation of Hume.