Rotatory motion

Rotatory motion is an infinite motion that is single and continuous.

The motion of everything that is in process of locomotion is either:

• rotatory
• rectilinear or
• a compound of the two

Consequently, if one of the former two is not continuous, that which is composed of them both cannot be continuous either.

If the locomotion of a thing is rectilinear and finite it is not continuous locomotion.

• The thing must turn back
• That which turns back in a straight line undergoes two contrary locomotions, since, so far as motion in respect of place is concerned:
  • upward motion is the contrary of downward motion
  • forward motion of backward motion
  • motion to the left of motion to the right

These are the pairs of contraries in the sphere of place.

But we have already defined single and continuous motion to be motion of a single thing in a single period of time. It operates within a sphere admitting of no further specific differentiation.

We have 3 things to consider:

1. The thing in motion

e.g. a man or a god

2. The ‘when’ of the motion, the time

3. The sphere within which it operates

This may be either place or affection or essential form or magnitude)

Contraries are specifically not one and the same but distinct.

Within the sphere of place, we have the above-mentioned distinctions.

We have an indication that motion from A to B is the contrary of motion from B to A in the fact that, if they occur at the same time, they arrest and stop each other.

The same is true in the case of a circle.

The motion from A towards B is the contrary of the motion from A towards G. Even if they are continuous and there is no turning back they arrest each other, because contraries annihilate or obstruct one another.

On the other hand lateral motion is not the contrary of upward motion.

Rectilinear motion cannot be continuous.

This is because turning back necessarily implies coming to a stand when locomotion is in:

• a straight line that is traversed
• a circle.
  • This is not the same thing as rotatory locomotion. When a thing merely traverses a circle, it may either proceed on its course without a break or turn back again when it has reached the same point from which it started

This coming to a stand is proven by observation and theory.

We have 3 points:

1. Starting-point
2. Middle-point
3. Finishing-point

The middle-point in virtue of the relations in which it stands severally to the other two is both a starting-point and a finishing-point, and though numerically one is theoretically two.

The distinction between the potential and the actual.

So in the straight line in question any one of the points lying between the two extremes is potentially a middle-point: but it is not actually so unless that which is in motion divides the line by coming to a stand at that point and beginning its motion again:

Thus the middle-point becomes both a starting-point and a goal, the starting-point of the latter part and the finishing-point of the first part of the motion.

This is the case for example when A in the course of its locomotion comes to a stand at B and starts again towards G.

But when its motion is continuous A cannot either have come to be or have ceased to be at the point B.

It can only have been there at the moment of passing, its passage not being contained within any period of time except the whole of which the particular moment is a dividing-point.

To maintain that it has come to be and ceased to be there will involve the consequence that A in the course of its locomotion will always be coming to a stand: for it is impossible that A should simultaneously have come to be at B and ceased to be there, so that the two things must have happened at different points of time, and therefore there will be the intervening period of time.

Consequently, A will be in a state of rest at B, and similarly at all other points, since the same reasoning holds good in every case.

When to A, that which is in process of locomotion, B, the middle-point, serves both as a finishingpoint and as a starting-point for its motion, A must come to a stand at B, because it makes it two just as one might do in thought.

However, the point A is the real starting-point at which the moving body has ceased to be, and it is at G that it has really come to be when its course is finished and it comes to a stand. So this is how we must meet the difficulty that then arises, which is as follows.