And so it is plain our senses are prone to deceive us. Hence arises the fourth great question of Greek philosophy: Have we any criterion of truth? [Sidenote: Importance of the views of Pythagoras.] The moment a suspicion that we have not crosses the mind of man, he realizes what may be truly termed intellectual despair. Is this world an illusion, a phantasm of the imagination? If things material and tangible, and therefore the most solid props of knowledge, are thus abruptly destroyed, in what direction shall we turn? Within a single century Greek philosophy had come to this pass, and it was not without reason that intelligent men looked on Pythagoras almost as a divinity upon earth when he pointed out to them a path of escape; when he bid them reflect on what it was that had thus taught them the fallibility of sense. For what is it but reason that has been thus warning us, and, in the midst of delusions, has guided us to the truth--reason, which has objects of her own, a world of her own? Though the visible and audible may deceive, we may nevertheless find absolute truth in things altogether separate from material nature, particularly in the relations of numbers and properties of geometrical forms. There is no illusion in this, that two added to two make four; or in this, that any two sides of a triangle taken together are greater than the third. If, then, we are living in a region of deceptions, we may rest assured that it is surrounded by a world of truth. [Sidenote: Influence of the Eleatic school and the Sophists.] From the material basis speculative philosophy gradually disengaged itself through the labours of the Eleatic school, the controversy as to the primary element receding into insignificance, and being replaced by investigations as to Time, Motion, Space, Thought, Being, God. The general result of these inquiries brought into prominence the suspicion of the untrustworthiness of the senses, the tendency of the whole period being manifested in the hypothesis at last attained, that atoms and space alone exist; and, since the former are mere centres of force, matter is necessarily a phantasm. When, therefore, the Athenians themselves commenced the cultivation of philosophy, it was with full participation in the doubt and uncertainty thus overspreading the whole subject. As Sophists, their action closed this speculative period, for, by a comparison of all the partial sciences thus far known, they arrived at the conclusion that there is no conscience, no good or evil, no philosophy, no religion, no law, no criterion of truth. [Sidenote: Age of faith--its solutions.] [Sidenote: Its continuation by Plato, and its end by the Sceptics.]