According to Democritus, "Nothing is true, or, if so, is not certain to us." Nevertheless, as, in his system sensation constitutes thought, and, at the same time, is but a change in the sentient being, "sensations are of necessity true;" from which somewhat obscure passage we may infer that, in the view of Democritus, though sensation is true subjectively, it is not true objectively. The sweet, the bitter, the hot, the cold, are simply creations of the mind; but in the outer object to which we append them, atoms and space alone exist, and our opinion of the properties of such objects is founded upon images emitted by them falling upon the senses. Confounding in this manner sensation with thought, and making them identical, he, moreover, included Reflexion as necessary for true knowledge, Sensation by itself being untrustworthy. Thus, though Sensation may indicate to us that sweet, bitter, hot, cold, occur in bodies, Reflexion teaches us that this is altogether an illusion, and that, in reality, atoms and space alone exist. [Sidenote: He introduces the atomic theory.] [Sidenote: Destiny, Fate and resistless law.] Devoting his attention, then, to the problem of perception--how the mind becomes aware of the existence of external things--he resorted to the hypothesis that they constantly throw off images of themselves, which are assimilated by the air through which they have to pass, and enter the soul by pores in its sensitive organs. Hence such images, being merely of the superficial form, are necessarily imperfect and untrue, and so, therefore, must be the knowledge yielded by them. Democritus rejected the one element of the Eleatics, affirming that there must be many; but he did not receive the four of Empedocles, nor his principles of Love and Hate, nor the homoeomeriæ of Anaxagoras. He also denied that the primary elements had any sensible qualities whatever. He conceived of all things as being composed of invisible, intangible, and indivisible particles or atoms, which, by reason of variation in their configuration, combination, or position, give rise to the varieties of forms: to the atom he imputed self-existence and eternal duration. His doctrine, therefore, explains how it is that the many can arise from the one, and in this particular he reconciled the apparent contradictions of the Ionians and Eleatics. The theory of chemistry, as it now exists, essentially includes his views. The general formative principle of Nature he regarded as being Destiny or Fate; but there are indications that by this he meant nothing more than irreversible law. [Sidenote: Is led to atheism.]