The essay “On the Five Essences” treats the ideas of the five conditions of matter, form, movement, time, and place. Of these he defines (a) matter as that which receives the other essences but cannot itself be received as an attribute, and so if the matter is taken away the other four essences are necessarily removed also. (b) Form is of two kinds, that which is the essential of the genius, being inseparable from the matter, and that which serves to describe the thing itself, i.e., the ten Aristotelian categories--substance, quantity, quality, relation, place, time, situation, condition, action, and passion; and this form is the faculty whereby a thing (_shay´_) is produced from formless matter, as fire is produced from the coincidence of dryness and heat, the matter being the dryness and heat, the form being the fire; without form the matter is abstract but real, becoming a thing when it takes form. As De Vaux points out (_Avicenne_, p. 85) this illustration shows that al-Kindi does not grasp Aristotle’s meaning correctly. (c) Movement is of six kinds: two are variations in substance, as either generation or corruption, i.e., production or destruction; two are variations in quantity by increase or decrease; one is variation in quality, and one is change of position. (d) Time is itself akin to movement, but proceeds always and only in one direction; it is not movement, though akin, for movement shows diversities of direction. Time is known only in relation to a “before” or “after,” like movement in a straight line and at a uniform rate, and so can only be expressed as a series of continuous numbers. (e) Place is by some supposed to be a body, but this is refuted by Aristotle: it is rather the surface which surrounds the body. When the body is taken away the place does not cease to exist, for the vacant space is instantly filled by some other body, air, water, etc., which has the same surrounding surface. Admittedly al-Kindi shows a crude treatment of these ideas, but he was the first to direct Arabic thought in this direction, and from these arose a new attitude towards the revealed doctrine of creation on the part of those who came after him. Al-Kindi, the “Philosopher of the Arabs,” as he was called (circ. 365), contains our best account of the various sects existing in Islam towards the end of the 3rd century A.H. as he met them in the course of his travels. It has been published as the second volume of De Goeje’s _Bibliotheca Geographorum Arab_. (Leiden., 1873).