Motion is the realization of the potential _qua_ potential. This is an Aristotelian definition and applies not merely to motion in the strict sense, _i. e._, movement in place, or motion of translation, but embraces all kinds of change. Take as an example the warming of the air in a cold room. The process of heating the room is a kind of motion; the air passes from a state of being cold to a state of being warm. In its original state as cold it is potentially warm, _i. e._, it is actually not warm, but has the capacity of becoming warm. At the end of the process it is actually warm. Hence the process itself is the actualization of the potential. That which is potential cannot make itself actual, for to make itself actual it must be actual, which is contrary to the hypothesis of its being potential. Potentiality and actuality are contradictory states and cannot exist side by side in the same thing at the same time in the same relation. There must therefore be an external agent, itself actual, to actualize a potential. Thus, in the above illustration, a cold room cannot make itself warm. There must be some agency itself actually warm to cause the air in the room to pass from cold to warm. This is true also of motion in place, that a thing cannot move itself and must be moved by something else. But that something else if itself in motion must again be moved by something else. This process would lead us to infinity. In order that a given thing shall be in motion, it would be necessary for an infinite number of things to be in motion. This is impossible, because there cannot be an infinite number of things all here and now. It is a contradiction in terms. Hence if anything is to move at all, there must be at the end of the finite chain a link which while causing the next link to move, is itself unmoved. Hence the motion existing in the world must be due ultimately to the existence of an unmoved mover. If this being causes motion without being itself in motion it does not act upon the bodies it moves as one body acts upon another, for a body can move another body only by being itself in motion. The manner in which the unmoved mover moves the world is rather to be conceived on the analogy of a loved object moving the loving object without itself being moved. The person in love strives to approach and unite with the object of his love without the latter necessarily being moved in turn. This is the way in which Aristotle conceives of the cause of the world's motion. There is no room here for the creation of the world. Matter is eternal, motion is eternal, and there is an eternal mind for the love of which all motions have been going on, eternally.