Aristotle’s Physics Book III

Change is the genus of motion/movement. Since movement is found in all of the categories, and since what is in all of the categories must transcend them, then movement transcends the categories and is prior. That leaves only potency and act to define it.

A body in full act cannot move and neither is one in potency since is hasn’t started moving. So “insofar as it is in potential” indicates it is still moving. Formal def.

Motion does not belong to any category of being since it is becoming. But since only three categories can terminate motion, and since motion can be reduced to the category in which it terminates, motion can be reduced to quality (alteration), quantity (diminution), and substance (generation/corruption).

Qualitative Types/Parts of Motion:

• Point/place departure to point/place of arrival between contraries is localmotion
• Alteration is only in the third species of quality – sensible qualities. Substantial changes are preceded by changes that dispose matter toward becoming a new being.
• Augmentation does not reduce to mere addition of quantities to form an aggregation…this would reduce to localmotion. Alteration takes place within the unity of a single substance, which happens only within living beings. The body expands spatially, and from contrary to contrary.
• Action and passion are not different types of motion, but rather motion considered from as “from agent” or the same motion as  “to” or “in patient”

Quantitative Types/Parts of Motion:

• Localmotion produces extension (magnitude) between contraries, manifests a continuum.
• Motion is a continuum because it is a magnitude (reveals place). Time is a continuum, because it measures motion – whose successive parts are known only through representations in memory.
• This quantitative aspect of motion also relates to infinity.

Book III, Chapter 1 – Formal Definition of Motion

He defines motion as the actualization of potential qua potential. The definition is difficult to understand at first, especially to someone who does not have a clear concept of act and potency, since these are about the most abstract concepts that exist. Act and potency are the contraries of every category, and as established in Book I, all change is between contraries. So in an even more fundamental sense, all change is between the contraries of act and potency in the 10 categories, insofar as the thing has potential to do so. The definition conveys that becoming is not something that already happened by adding “insofar as there is potential.” If it were simply the actualization of potential, there would be no potential left to actualize, which we don’t really see as long as change as taking place.

Book III, Chapter 2 – Material Definition of Motion

This is called the material definition and the first demonstration in natural philosophy: Since the actualization of potential insofar as there is potential is the movement is the act of the movable qual movable, and since motion is the actualization of potential inasfar as there is a potential, and since the

Book III, Chapter 3 – Action and Passion are One

Action and passion are either: 1) both in the moved and the mover, or 2) agency is the in agent and patience in the patient. Its neither of these though because if it were 1, a thing would have two simultaneous motions. If it were 2, then not everything moving is being moved. So Action is motion considered as from the agent, and passion is motion considered as from the patient…they are one in the same though, like road to Thebes.

Book III, Chapter 4 – The Infinite

Motion makes us think about the infinite, since it may have no end. With the definition of nature established, Aristotle moves on to the question of the infinite for spatial magnitudes, motion and time. Whether the infinite exists, and what it is are closely related to motion historically because of the philosophers who have treated it, and so Aristotle claims they should be treated here, even though qualities and magnitudes (perhaps qualities or a point in math), are numerical. Past philosophers have made it the principle of all things.  Various earlier opinions. Pythagoreans think the infinite is in the sensible world because what is outside heaven is infinite. Plato on the other hand thinks there is nothing outside the infinite and sensible things are made of it, but the forms are not since they are nowhere. The Pythagoreans identify the infinite with even numbers while earlier physicists, some who say the elements are not infinite in number like Thales or Anaxamander, while others make them infinite like Anaxagorus or Democritus (continuous by contact). Things that make us think of the infinite. They all make inifinte a principle or source, but Aristotle says calling it a principle limits it actually. Aristotle thinks 5 considerations typically bring the infinite to mind: 1) the nature of time continuing indefinitely, 2) the division of magnitudes 3) generation and corruption 4) if everyting is always limited by something different than itself, that something different is infinite 5) imagination of space. Senses: actual, mathematical, and potential infinite. The fifth one makes people think that body is also infinite and there are an infinite number of worlds, since if void is infinite, body must be also. If it exists, how does it exist? There are three senses infinite is used: 1) sense that voice is invisible (can’t be perceived by sight) 2) going through but no term (number) 3) naturally can go through but not actually (water cycle).

Book III, Chapter 5 – Actual infinite body can’t exist, because it would be actually indivisible.

Physics’ question is whether an infinite material body actually exists or not. There are reasons not to believe so: 1) from natural facts it appears that an unlimited body could not be either 1) a compound of a limited number of elements or 2) a simple uniform element like Anaximander. 1) If the infinite were one element unlimited it would devour the rest. 2) places would not exist because up and down, because a body must be in a place which is a limited thing…so no place can contain an unlimited body. 

Book III, Chapter 6 – If not actually, then how? As something that has no end (potential), since end is a limit.

But the infinite must exist somehow because time can’t have a beginning and end, nor can counting come to an end, or division of a line. There is a sense that infinite exists, and a sense that it doesn’t. A potential statue means it is coming to be but that is not the case with the infinite. The divisibility of spatial magnitude is potentially unlimited, but this will never be realized like the day coming into existence actually does, or the games actually does…it remains a “potential” statue, just like you can always take out one more, if you keep putting rocks back in a bag. The unlimited by addition is like the unlimited by way of division. Since one can always take a percent of something, in this sense a limited magnitude may be the actual seat of an illimitable process – the infinite turns out to be the contrary of what is said to exist…it is what always has something outside it. Nothing is complete that has no end; and the end is a limit. 

Definition: A quantity is infinite if it is such that we can always take a part outside what has been already taken.

Nothing is complete which has no end, and the end is a limit. The infinite is beyond any definable limit, its essence is incompleteness. The unlimited is analogous to the matter from which the whole is made by form. A line is actual, but it has a potentially infinite number of points. The same is true with motion – when a motion is viewed without terminating points, it is potentially infinite and divisible a characteristic of potential. 

Book III, Chapter 7