By this abrupt contraposition of body and mind as reciprocally independent 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 not come from ourselves, is shown by the fact that it is not in our power to determine the objects which we perceive, or the character of our perception of them.  The supposition that God has caused our perceptions directly, or by means of something which has no resemblance whatever to an external object extended in three dimensions and movable, is excluded by the fact that God is not a deceiver.  In reliance on God’s veracity we may accept as true whatever