The principle of the One is altogether ideal and belongs entirely to thought, even though we wish to say that atoms exist. The atom may be taken materially, but it is supersensuous, purely intellectual. In our times, too, more especially through the instrumentality of Gassendi, this conception of atoms has been renewed. The atoms of Leucippus are, however, not molecules, the small particles of Physics. In Leucippus, according to Aristotle, (De gen. et corr. I. 8) there is to be found the idea that “atoms are invisible because of the smallness of their body,” which is much like the way in which molecules are now-a-days spoken of: but this is merely a way of speaking of them. The One can neither be seen nor shown with magnifying glasses or measures, because it is an abstraction of thought; what is shown is always matter that is put together. It is just as futile when, as in modern times, men try by the microscope to investigate the inmost part of the organism, the soul, and think they can discover it by means of sight and feeling. Thus the principle of the One is altogether ideal, but not in the sense of being in thought or in the head alone, but in such a way that thought is made the true essence of things. Leucippus understood it so, and his philosophy is consequently not at all empirical. Tennemann (Vol. 1, p. 261), on the other hand, says, quite wrongly, “The system of Leucippus is opposed to that of the Eleatics; he recognizes the empirical world as the only objective reality, and body as the only kind of existence.” But the atom and the vacuum are not things of experience; Leucippus says that it is not the senses through which we become conscious of the truth, and thereby he has established an idealism in the higher sense and not one which is merely subjective. _b. The Constitution of the World_