Summary: Physics (page 3)

Luck is happy chance.

The accidental cause (chance) cannot precede the intrinsic cause (intellect or nature): It is necessary for intellect and nature to be the [first] causes of this universe ¹ (prior to chance).

Since Aristotle defines nature as principle of motion and stability ², it follows that unmoved things no longer fall under nature ³.

There is finality in nature:

If it is by nature and with a view to an end that the swallow makes its nest, [...] it is manifest that such a cause exists in things that happen by nature ⁴.

Monsters — such as deformed children — provide further evidence of this: Monsters are errors of that which comes about with a view to an end ⁵.

Book III

What is movement? This question concerns the physicist, since nature — the object of his study — is a principle of motion.

An object in motion is called a mobile, and that which causes its motion is a mover: The mover is mover of the mobile and the mobile mobile under the action of the mover ⁶.

Since motion cannot exist outside of things, there are as many species of motion as of being ⁷.

Thus, the motion of what is alterable is alteration; the motion of what can increase is growth; and the motion of what is generable and perishable is generation and decay.

Aristotle uses the notion of entelechy to define movement. Some beings do not yet exist in actuality but exist potentially. Thus, the machine a scientist is designing does not yet exist in actuality but exists in potentiality — within his calculations.

Aristotle defines entelechy as the transition from potentiality to actuality. In other words, a being is in entelechy when it attains its proper end.

We can define movement through this notion of entelechy. Indeed, the entelechy of the being in potency, as such, is movement ⁸.

Let us take an example: When the constructible, insofar as we say it to be such, is in entelechy, it is in the process of being constructed, and this is construction ⁹.

We can also say that the entelechy of what is possible, as possible, it is manifest that it is movement ¹⁰.

Aristotle now turns to the study of the infinite. This is because the objects of study in physics — magnitudes, motion, and time — are each either finite or infinite. Infinity, therefore, is also one of the notions that physics must analyse.

According to the Pythagoreans, infinity is a reality in itself, existing in actuality within sensible things.

Where does our idea of infinity originate? According to Aristotle, it arises from six sources:

- Time, because it is unending

- The division of magnitudes

- The endless cycle of generation and perishing, made possible by infinity

- The concept of the unlimited: nothing is limited if everything can only be defined in relation to something else

- The infinity of numbers

- The idea of an infinite body

However, the actual existence of infinity leads to many impossibilities.

Infinity cannot exist in sensible things, for an infinite body would require an infinite space — something we never observe. Moreover, a body, by definition, is bounded by a surface and is therefore finite.

Therefore, infinity can exist only accidentally. In other words, the infinite exists only potentially, never in actuality.

Infinity can be defined negatively: It is that which cannot be traversed ¹¹, or more precisely, it is that which by nature cannot be traversed, and has neither end nor limit ¹².

¹ ibid. 198a
² Book II, 192b
³ ibid., 198a
⁴ ibid. 199a
⁵ ibid., 199b
⁶ III, 200b
⁷ ibid., 201a
⁸ ibid.
⁹ ibid., 201a
¹⁰ ibid., 201b
¹¹ ibid., 204a
¹² ibid.