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Chapter 1
When anything changes, it changes in one of three ways. The change can be a secondary or associated change, as when a musically giften man walks by and we say, "There goes musical talent." The motion of the musical talent is secondary to the motion of the man; it moves by being asociated with the man who is really moving. Or a change can be a generalized change, as when we ascribe to the whole of an object a change that occurs in a part. For example, when a sore throat or an eye ailment is cured, we say that the body has been cured, although the eye and the throat are but parts of the whole body. Yet something must change to have a secondary or generalized change, something primary that changes neither by association nor by generalization. We call this something by the technical term mobile (plural mobilia), signifying something that simply moves or changes, i.e., undergoes a primary change. (We will use the term "move" interchangeably with "change".) Such a primary mobile will differ with different types of change, e.g., a change in some quality or a change in some position. There are also different types of qualities; for example, a change in health is not the same as a change in warmth.

We define another technical term motor (plural motores) as something that causes a mobile to change. Motores can likewise be divided into three types: secondary, generalized and primary. A secondary motor causes change by associating with a primary motor, whereas a generalized motor contains a primary motor within itself. A primary motor is something that intrinsically causes a change, e.g., a doctor who heals or a fist that bruises.