In Metaphysics Theta 5 Aristotle talks about actual and potential capacities. Actual and potential capacities come in pairs. A passive capacity is the capacity of something to be affected by something else in a certain way. For example, water has the passive capacity to be heated by fire. An active capacity is a capacity a thing or a creature has, to change something or affect something, like fire has the active capacity of heating water. To Aristotle, capacities always come in pairs, like the capacity of the water to be heated by fire comes together with the capacity of fire to heat water. Also, water actually gets heated by fire only when nothing prevents the fire from heating the water. That means when a pair of capacities meet in the right conditions, the change from potentiality to actuality happens naturally.
In Metaphysics H2, Aristotle talks about potentiality and actuality. Taking the example we used above; water has the potentiality to be heated by fire. It becomes an actuality the moment water gets heated by fire. Or if I have the potentiality to play the flute, it only becomes an actuality when I am playing the flute at that moment.
Aristotle further states that there are innate capacities, but also capacities we can learn and that grow by habit. An example for the first would be the sight - we don’t need to learn how to see - and one for the latter would be playing the flute, which requires a certain amount of training. He continues affirming, that the difference between the two is that the first is the kind of capacities everything intrinsically possesses. The capacities that, when they meet in the right conditions, happen unless something is preventing them from doing so. For example, fire heats water unless water is protected through a heat-proof container. Those capacities differ from capacities rational creatures have. The latter type of capacities allows a person to exercise two contradictory things with the same skill. For example, when a medical student learns how to heal a sick person, he at the same time gets the knowledge for harming a client, because he would at the meantime learn how to contaminate him with an illness.
Following what has just been said, rational creatures would be capable of producing contradictory outcomes with the same capacity at the same time, because with the capacity the doctor has of healing, the contrary – the harming – comes together. Aristotle continues, that it is impossible to produce contradictory outcomes at the same time. A doctor cannot heal and harm his client at once. So, what decides over whether the doctor will harm or heal his client is his desire to do the one and not the other. Therefore, what is necessary for a rational capacity to become actual, is the decision, the desire of the agent to act in that certain way. Non-rational capacities do not have this aspect, they change from potentialities to actualities the moment nothing prevents them from doing so.
The problem of generality in capacities
There seem to be several problems with this perspective on capacities. One problem is that when a capacity cannot be exercised because of something preventing it, the capacity is another. For example, the sun has the active capacity of bleaching hair and hair has the passive capacity of being bleached by the sun. What prevents the sun from bleaching the hair would be, that the hair is for example protected by a hat. And then the sun’s capacity would not just be « to bleach hair », but « to bleach hair when it is not protected by a hat ». There are conditions that make the capacity impossible to happen and there are other conditions, that support the capacity to become actual. The notion of capacities raises therefore the question, what type of conditions would have to be included, - respectively be excluded - from the description of a capacity. This is a problem that comes up as soon as one distinguishes active capacities from passive capacities.
The second problem I want to mention comes up specially when one distinguishes between rational capacities and non-rational capacities. As mentioned above, the rational capacities are different from the non-rational, as the action happens when the agent decides to make it happen, not only when the active and the passive parts are in the right conditions. That implies that anytime a person makes a choice, it is a rational capacity that comes to happen. Capacities of rational beings have contradictory possible outcomes this is why choice is important in rational capacities. But first, if for example the doctor decides to heal his patient, but because he makes a mistake (for example he did not know the patient was allergic to a certain medicament, but gives it to him), even if his intention was to help him, he would have made his patient sick. In this case we would have to say that the doctor has the capacity of healing if he is not making mistakes. We would have to exclude almost an infinite number of mental circumstances, so that the rational capacity can happen. Things like the doctor must be concentrated, the doctor should have slept well and so on, that prevent the capacity on a mental level to happen. This brings us back to the problem stated before: what external conditions do we include in the description of a capacity? And in the case of rational capacities; what mental conditions do we include or exclude from its nature?
What was mentioned above is also called the problem of generality. This means, that we do not know how much to include in the definition of capacity. If we say:
A : « the doctor has the capacity of healing as long as nothing prevents him from doing so »,
there is a large number of points included in the notion « if nothing prevents him», for instance that he has slept well. Affirmation A becomes then very unspecific. But if we would include those circumstances that prevent the doctor from healing his client, the Affirmation A would then again become so specific, that it won’t be a universal description of how a capacity becomes actual anymore.
This means that doctor X would have other things preventing him from healing his patient than doctor Y. And so, we would have to make up a different description of how the capacity becomes actual for each doctor. In this case then, there is no universal capacity that doctor X and doctor Y would share, which is problematic, since that was Aristotle’s goal of his writing.
In a next step, we could have looked at other questions that come up, when distinguishing between rational capacities and non-rational capacities. One could be : is the desire of the doctor to heal his patient not also only a « favorable circumstance » ? This point questions whether the distinction of rational and non-rational is even necessary.
In this essay we first looked at the question what a capacity to Aristotle is. In the second part, we found that there is an important problem with his view, namely the problem of generality.
C.D.C. Reeve, 2016 : Aritstotles Metaphysics