s. It is true the external world makes its presence known
directly, when it breaks in upon the soul in sense-perception. But
Descartes's rationalism and love of mathematics forbade his attaching
importance to this criterion. Real nature, that exactly definable and
predictable order of moving bodies defined in physics, is not known
through sense-perception, but through thought. Its necessities are the
necessities of reason. Descartes finds himself, then, in the perplexing
position of seeking an internal criterion for an external world. The
problem of knowledge so stated sets going the whole epistemological
movement of the eighteenth century, from Locke through Berkeley and Hume
to Kant. And the issue of this development is the absolute idealism of
Kant's successors.

[Sidenote: Empirical Reaction of the English Philosophers.]

Sect. 186. Of the English philosophers who prepare the way for the
epistemology of Kant, Hume is the most radical and momentous. It was he
who roused Kant from his "dogmatic slumbers" to the task of the
"Critical Philosophy." Hume is one of the two possible consequences of
Descartes. One who attaches greater importance to the rational
necessities of science than to its external reference, is not unwilling
that nature should be swallowed up in mind. With Malebranche,
Descartes's immediate successor in France, nature is thus provided for
within the archetypal mind of God. With the English philosophers, on the
other hand, externality is made the very mark of nature, and as a
consequence sense-perception becomes the criterion of scientific truth.
This empirical theory of knowledge, inaugurated and developed by Locke
and Berkeley, culminates in Hume's designation of the _impression_ as
the distinguishing element of nature, at once making up its content and
certifying to its externality. The processes of nature are successions
of impressions; and the laws of nature are their uniformities, or the
expectations of uniformity which their repetitions engender. Hume does
not hesitate to draw the logical conclusion. If the final mark of truth
is the presence to sense of the individual element, then science can
consist only of items of information and probable generalizations
concerning their sequences. The effect is observed to follow upon the
cause in fact, but there is no understanding of its necessity; therefore
no absolute certainty attaches to the future effects of any cause.

[Sidenote: To Save Exact Sc