Page:Leibniz Discourse on Metaphysics etc (1908).djvu/8

 When Descartes, in the first half of the seventeenth century, said that there are only two kinds of things or substances in nature, namely, extended substances and thinking substances, or bodies and spirits; that, in bodies, everything is reducible to extension with its modifications of form, divisibility, rest and motion, while in the soul everything is reducible to thinking with its various modes of pleasure, pain, affirmation, reasoning, will, etc.…; when he in fact reduced all nature to a vast mechanism, outside of which there is nothing but the soul which manifests to itself its existence and its independence through the consciousness of its thinking, he brought about the most important revolution in modern philosophy. To understand its significance however an account must be given of the philosophical standpoint of the time.

In all the schools at that time the dominant theory was that of the Peripatetics, altered by time and misunderstood, the theory of substantial forms. It posited in each kind of substance a special entity which constituted the reality and the specific difference of that substance independently of the relation of its parts. For example, according to a Peripatetic of the time, "fire differs from water not only through the position of its parts but through an entity which belongs to it quite distinct from the materials. When a body changes its condition, there is no change in the parts, but one form is supplanted by another." Thus, when water becomes ice, the Peripatetics claimed that a new form substituted itself in place of the preceding form to constitute a new body. Not only did they admit primary or basal entities, or substantial forms to explain the differences in substances, but for small changes also, and for all the sensible qualities they had what were called accidental forms: thus hardness, heat, light were beings quite different from the bodies in which they were found.