Defining Potency

‘Potency’ has many meanings:

1. A source of movement or change in a thing

For example, the art of building is a potency which is not in the thing built.

The art of healing is a potency. It may be in the man healed. But it is not in him after he is healed.

Thus, ‘potency’ generally means the source of change or movement.

2. The source of a thing being moved by another thing

A patient who suffers anything is ‘capable’ of suffering. This ‘potency’ is a capability.

3. The capacity of performing this well or according to intention

Sometimes, we say that a person who cannot walk or speak well cannot walk or speak.

4. Passivity

5. An absolutely impassive or unchangeable state

This is when a thing cannot be easily changed for the worse.

Things are broken, crushed, and bent and in general destroyed by not having a potency.

Things are impassive when they are not affected or not so much affected by such breaking, crushing, and bending.

• This is due to a ‘potency’ that puts them in some positive state.
• This lets the do something.

Potency as Capability

Likewise, ‘potent’ or ‘capable’ in one sense means that which can begin a movement or a change in general.

• Even that which can bring things to rest is a ‘potent’ thing in itself.

Even that which perishes is ‘capable’ of perishing.

• A thing would not have perished if it had not been capable of perishing.
• It has a certain disposition, cause, and principle which fits it to suffer perishing.

Some people think that it perishes because it has something.

• Others think that it perishes because it lacks something.

In this way, ‘capable’ is used in 2 senses:

1. Lack is ‘habit’

In this case, everything will be capable by having something.

• In this way, things becaome capable both by having a positive habit and by principle [of potency]
• The lack of potency then leads to lack.

2. Lack is not ‘habit’

In this case, a thing is capable because it or any other thing has a potency or principle which can destroy it.

• It is capable either because the thing might merely chance to happen or not to happen

This sort of potency is found even in lifeless things such as instruments.

We say one lyre can speak if it has a good tone. It cannot speak at all if it does not have a good tone.

Incapacity

Incapacity is lack of capacity.

It is used for things that would naturally have the capacity, but does not have it.

For example, a eunuch is ‘incapable of begetting’. But it is a man which is usually capable of begetting.

To either kind of capacity there is an opposite incapacity.

1. One incapacity prevents movement

This incapacity is called adunata

2. Another incapacity prevents good movement

Adunata

The opposite of ‘impossible’ is that which is necessarily true

• For example, it is impossible for the diagonal of a square to be commensurate with its side.
  • The contrary of this is the possible.

‘Possible’, in one sense, means that which is not necessarily false.

• For example, it is possible for man to sit down.
  • A man that is not seated is not necessarily false.

A ‘potency’ or ‘power’ in geometry is so called by a change of meaning.

These senses of ‘capable’ or ‘possible’ involve no reference to potency.

But the senses which involve a reference to potency all refer to the primary kind of potency.

This is a source of change in the thing or in another thing.

Some people call things as ‘capable’ when:

• something else has a potency over them, or
• something else has no potency over them
• something else has a potency over them in a particular way.

The same is true of the things that are incapable.

Therefore, the proper definition of the primary kind of potency is: ‘a source of change in another thing or in the same thing qua other’.