substances, Descartes founded that dualism, as whose typical representative
he is still honored or opposed. This dualism between the material and
spiritual worlds belongs to those standpoints which are valid without being
ultimate truth; on the pyramid of metaphysical knowledge it takes a high,
but not the highest, place. We may not rest in it, yet it retains a
permanent value in opposition to subordinate theories. It is in the
right against a materialism which still lacks insight into the essential
distinction between mind and matter, thought and extension, consciousness
and motion; it loses its validity when, with a full consideration and
conservation of the distinction between these two spheres, we succeed in
bridging over the gulf between them, whether this is accomplished through
a philosophy of identity, like that of Spinoza and Schelling, or by an
idealism, like that of Leibnitz or Fichte. In any case philosophy retains
as an inalienable possession the negative conclusion, that, in view of the
heterogeneity of consciousness and motion, the inner life is not reducible
to material phenomena. This clear and simple distinction, which sets bounds
to every confusion of spiritual and material existence, was an act of
emancipation; it worked on the sultry intellectual atmosphere of the time
with the purifying and illuminating power of a lightning flash. We shall
find the later development of philosophy starting from the Cartesian
dualism.

Descartes himself looked upon the fundamental principles which have now
been discussed as merely the foundation for his life work, as the entrance
portal to his cosmology. Posterity has judged otherwise; it finds his chief
work in that which he considered a mere preparation for it. The start from
doubt, the self-certitude of the thinking ego, the rational criterion of
certitude, the question of the origin of ideas, the concept of substance,
the essential distinction between conscious activity and corporeal being,
and, also, the principle of thoroughgoing mechanism in the material world
(from his philosophy of nature)--these are the thoughts which assure his
immortality. The vestibule has brought the builder more fame, and has
proved more enduring, than the temple: of the latter only the ruins remain;
the former has remained undestroyed through the centuries.

%2. Nature.%

What guarantee have we for the existence of material objects affecting our
senses? That the ideas of sense do