All the essential constituents of the modern mechanical theories are really to be found in Democritus, the causal interpretation, the denial of any operative purposes or formative principles, the admission and assertion of quantitative explanations alone, the denial of qualities, the reduction of all cosmic developments to the “mechanics of the atom” (even to attractions and repulsions, thus setting aside the “energies”), the inevitable necessity of these mechanical sequences, indeed at bottom even the conviction of the “constancy of the sum of matter and energy.” (For, as he says, “nothing comes out of nothing.”) And although he makes the “soul” the principle of the phenomena of life, that is in no way contradictory to his general mechanical theory, but is quite congruent with it. For the “soul” is to him only an aggregation of thinner, smoother, and rounder atoms, which as such are more mobile, and can, as it were, quarter themselves in the body, but nevertheless stand in a purely mechanical relation to it. Aristotle, who was well aware of the diametrical opposition, represents, as compared with Democritus, the Socratic-Platonic teleological interpretation of nature, and in regard to the question of living organisms his point of view may quite well be designated by the modern name of “vitalism.” Especially in his theory of the vegetable soul, the essence of vitalism is already contained. It is the λόγος ἐνυλος (logos enhylos), the idea immanent in the matter, the conceptual essence of the organism, or its ideal whole, which is inherent in it from its beginnings in the germ, and determines, like a directing law, all its vegetative processes, and so raises it from a state of “possibility” to one of “reality.” All that we meet with later as “nisus formativus,” as “life-force” (vis vitalis), as “endeavour after an end” (Zielstrebigkeit), is included in the scope of Aristotelian thought. And he has the advantage over many of his successors of being very much clearer.(57) The present state of the problem of life may be regarded as due to a reaction of biological investigation and opinion from the “vitalistic” theories which prevailed in the first half of last century, and which were in turn at once the root and the fruit of the German Nature-philosophy of that time.