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Everything that exists potentially and whose essence includes a certain state of possibility, may at some time be without actual existence.

That which is potentially a certain thing is necessarily material, for the state of possibility is always connected with matter.

Each compound substance consists of matter and form, and requires an agent for its existence, viz., a force which sets the substance in motion, and thereby enables it to receive a certain form. The force which thus prepares the substance of a certain individual being, is called the immediate motor.

Here the necessity arises of investigating into the properties of motion, the moving agent and the thing moved. But this has already been explained sufficiently; and the opinion of Aristotle may be expressed in the following proposition: Matter does not move of its own accord-an important proposition that led to the investigation of the Prime Motor (the first moving agent).

Of these foregoing twenty-five propositions some may be verified by means of a little reflection and the application of a few propositions capable of proof, or of axioms or theorems of almost the same force, such as have been explained by me. Others require many arguments and propositions, all of which, however, have been established by conclusive proofs partly in the Physics and its commentaries, and partly in the Metaphysics and its commentary. I have already stated that in this work it is not my intention to copy the books of the philosophers or to explain difficult problems, but simply to mention those propositions which are closely connected with our subject, and which we want for our purpose.

To the above propositions one must be added which enunciates that the universe is eternal, and which is held by Aristotle to be true, and even more acceptable than any other theory. For the present we admit it, as a hypothesis, only for the purpose of demonstrating our theory. It is the following proposition:

Time and motion are eternal, constant, and in actual existence.

In accordance with this proposition, Aristotle is compelled to assume that there exists actually a body with constant motion, viz., the fifth element. He therefore says that the heavens are not subject to genesis or destruction, because motion cannot be generated nor destroyed. He also holds that every motion must necessarily be preceded by another motion, either of the same or of a different kind. The belief that the locomotion of an animal is not preceded by another motion, is not true: for the animal is caused to move, after it had been in rest, by the intention to obtain those very things which bring about that locomotion. A change in its state of health, or some image, or some new idea can produce a desire to seek that which is conducive to its welfare and to avoid that which is contrary. Each of these three causes