Substance

Descartes, etc.

The Aristotelian conception of substance de termined for many centuries the form of western thought and metaphysics. Mediaeval Scholasticism was in its essence a phi losophy of "substantial forms." This fundamental outlook was shaken only by the new scientific view of the world. which origi nated in the 17th century. The establishment of the heliocentric system by Copernicus and Kepler, and of modern dynamics by Galilei, deprived the Aristotelian physics, cosmology, and meta physics, of their foundation. The new concept of substance goes back historically to pre-Aristotelian philosophy, especially to the doctrines of the Pythagoreans and of Democritus. Substance is now conceived mathematically; it is that which, in all transforma tions of phenomena, remains unchangeable in magnitude.

Thus, the problem of substance, or matter, is supplemented by the problem of motion. For also motion, now conceived as pure translocation, reveals a quantitative constancy which makes it something substantial. In this sense, the Galilean doctrine of motion is headed by the principal of inertia, i.e., the theorem that the motion of a material point upon which no external forces are working, remains invariant in velocity and direction. Similarly, the mechanics of Descartes rests on the principle of the "preser vation of motion," i.e., the assumption that the so-called magni tude of motion, which is measured by the product of mass and velocity (my), can suffer neither increase nor diminution. This product remains constant in the whole world ; through the ex change of velocity, which takes place at the impact of bodies, no new quantity of motion can be created nor any existing one be destroyed; there occurs only a change in the distribution.

From this assumption, however, it follows that the world of bodies is a system completely closed in itself and incapable of suffering any interference from the outside. All action of body upon body, all "causality" within the realm of extension, rests upon mechanical laws; mental, "immaterial" powers cannot act upon bodies, nor increase or diminish the sum total of their mo mentum. Between the worlds of "external" and "internal" ex perience, between "matter" and "consciousness," no kind of tran sition takes place.

Through this viewpoint, the metaphysical doctrine of substance receives a decisive turn. Descartes defines substance (Principia philosophae, i. 5o) as that type of thing which exists in such a way that it needs no other thing for its existence. Accordingly, the

concept of substance is applicable only to the Being of God. Next to it, there subsist, as relatively independent and mutually irre ducible entities, consciousness and the world of bodies. The fundamental problem consists now in determining how these two essentially different substances can enter that kind of connection which is exhibited in the concrete existence of man. For man is a whole consisting of "thought" and "extension," of "soul" and "body." The search for an explanation of the empirical union of soul and body, notwithstanding their metaphysical difference, gives rise to the systems of Spinozism, of Occasionalism, and of the doctrine of monads.

Locke and Hume.

A new and decisive turn is given to the problem by the critical analysis of cognition which starts with Locke and Hume and reaches its completion in Kant. The charac teristic of this turn lies in the fact that the weight of the problem is transferred from the realm of metaphysics to that of episte mology. Substance appears as a "category," as a fundamental con cept of pure understanding. That this concept is not derivable directly from experience, that it cannot be thought as the mere "copy" of a sense-impression : this is emphasised also by Locke and Hume.

The empiricist and sensualist conception of knowledge, which they represent, is thus confronted with the question whether the concept of substance, not being reducible to empirical sources, retains any "objective" significance or is merely "subjective" as a kind of natural illusion of the human understanding. Locke starts with the assumption that true reality belongs only to the simple, sensible "ideas," to the sensations of colour, sound, etc., whereas the understanding cannot create any new reality, but can merely connect these ideas in certain modes with one another. Such a mode of connection is represented by the concept of substance. What the senses deliver individually, is thereby connected ; the changing states and properties being united in one "substratum," or "bearer." But the idea of a persistent thing as "bearer" of the changeable qualities is, in itself, utterly empty; it represents a mere form of knowledge to which no content corresponds. Not adding anything new to the sensible elements in which all our knowledge of reality is rooted, it originates in the mere habit of correlating with one another many such elements on the basis of their regular conjunction, their spatial coexistence.