But naturalism becomes fundamentally different when it ceases to remain at the level of naïve or fancifully conceived ideas of “nature” and “natural occurrences,” when, instead of poetry or religious sentiments, it incorporates something else, namely, exact natural science and the idea of a mathematical-mechanical calculability in the whole system of nature. “Nature” and “happening naturally”, as used by the naïve intelligence, are half animistic ideas and modes of expression, which import into nature, or leave in it, life and soul, impulse, and a kind of will. And that speculative form of naturalism which tends to become religious develops this fault to its utmost. But a “nature” like this is not at all a possible subject for natural science and exact methods, not a subject for experiment, calculation, and fixed laws, for precise interpretation, or for interpretation on simple rational principles. Instead of the naïve, poetical, and half mystical conceptions of nature we must have a really scientific one, so that, so to speak, the supernatural may be eliminated from nature, and the apparently irrational rationalised; that is, so that all its phenomena may be traced back to simple, unequivocal, and easily understood processes, the actual why and how of all things perceived, and thus, it may be, understood; so that, in short, everything may be seen to come about “by natural means.” There is obviously one domain and order of processes in nature which exactly fulfils those requirements, and is really in the fullest sense “natural,” that is, quite easily understood, quite rational, quite amenable to computation and measurement, quite rigidly subordinate to laws which can be formulated. These are the processes of physics and chemistry, and in a still higher degree those of movement in general, the processes of mechanics in short. And to bring into this domain and subordinate to its laws everything that occurs in nature, all becoming, and passing away, and changing, all development, growth, nutrition, reproduction, the origin of the individual and of the species, of animals and of man, of the living and the not living, even of sensation and perception, impulse, desire and instinct, will and thought—this alone would really be to show that things “happen naturally,” that is, to explain everything in terms of natural causes. And the conviction that this can be done is the only true naturalism.