The philosophical systems, in spite of their number and variety, may be reduced to three: (1) the Materialists; (2) the Naturalists; (3) the Theists. (1) _The Materialists._ They reject an intelligent and omnipotent Creator and disposer of the universe. In their view the world exists from all eternity and had no author. The animal comes from semen and semen from the animal; so it had always been and will always be; those who maintain this doctrine are atheists. (2) _The Naturalists._ These devote themselves to the study of nature and of the marvelous phenomena of the animal and vegetable world. Having carefully analyzed animal organs with the help of anatomy, struck with the wonders of God's work and with the wisdom therein revealed, they are forced to admit the existence of a wise Creator who knows the end and purpose of everything. And certainly no one can study anatomy and the wonderful mechanism of living things without being obliged to confess the profound wisdom of him who has framed the bodies of animals and especially of man. But carried away by their natural researches they believed that the existence of a being absolutely depended upon the proper equilibrium of its organism. According to them, as the latter perishes and is destroyed, so is the thinking faculty which is bound up with it; and as they assert that the restoration of a thing once destroyed to existence is unthinkable, they deny the immortality of the soul. Consequently they deny heaven, hell, resurrection, and judgment. Acknowledging neither a recompense for good deeds nor a punishment for evil ones, they fling off all authority and plunge into sensual pleasures with the avidity of brutes. These also ought to be called atheists, for the true faith depends not only on the acknowledgment of God, but of his Apostle and of the day of judgment. And although they acknowledge God and his attributes, they deny a judgment to come. (3) Next come the _Theists_. Among them should be reckoned Socrates, who was the teacher of Plato as Plato was of Aristotle. This latter drew up for his disciples the rules of logic, organized the sciences, elucidated what was formerly obscure, and expounded what had not been understood. This school refuted the systems of the two others, _i.e._, the Materialists and Naturalists; but in exposing their mistaken and perverse beliefs, they made use of arguments which they should not. "God suffices to protect the faithful in war" (Koran, xxxiii. 25). Aristotle also contended with success against the theories of Plato, Socrates, and the theists who had preceded him, and separated himself entirely from them; but he could not eliminate from his doctrine the stains of infidelity and heresy which disfigure the teaching of his predecessors. We should therefore consider them all as unbelievers, as well as the so-called Mussulman philosophers, such as Ibn Sina (Avicenna) and Farabi, who have adopted their systems.