Why We Need Act and Potency

It is no longer common to explain the nature of change in terms of act and potency. However, the philosophers of antiquity, such as Aristotle, saw the act-potency distinction as being essential to explain change. Indeed, Aristotle saw the distinction as necessary to respond to the challenge of Parmenides, who denied the reality of change.  

Parmenides’ argument is that being can only change if made to do so by something other than it. But the only thing other than being is non-being. Hence, change would require being to arise out of non-being. However, since nothing can arise from non-being (nothingness), change is therefore illusionary. For suppose that we have a toy plastic car and I put a match to it; it will ultimately melt and turn into mushy goo. But what metaphysical explanation can we provide to explain what has given rise to this goo? The explanation cannot reside completely in the fire because not everything fire touches turns to goo. The explanation cannot be that the goo is a combination of the fire and toy car, for any combination of things can be separated back into their separate parts, even if just theoretically; yet, we do not find fire or a toy car as parts within the goo. And the goo obviously was not hiding somewhere within the car, so the only explanation left, argues Parmenides, is that the goo has come from nothing, which is to say that it has not come at all but is just illusionary.

It might seem that the best way to respond to a challenge like Parmenides would be to disregard his position as absurd and inconsistent. If change did not occur, then I could not have typed this blog post on the nature of act and potency, nor could Parmenides have gone through the necessary reasoning in his brain to arrive at the conclusion that change is illusionary. That change occurs is just self-evident. 

However, this way of responding is inadequate; for it tells us that something has gone wrong, but it does not tell us what has gone wrong. It is only once we appeal to Aristotle’s act-potency distinction that we can see where Parmenides has erred. Aristotle argued that the error in Parmenides reasoning is to think that he has exhausted all possibilities as to an explanation for the rise of change. Aristotle maintained that there is another available analysis of change, on which change does not arise out of non-being but rather out of being of another kind; namely, being-in-potency - the way a thing could potentially be. To illustrate being-in-potency, a bar of gold is an example of being-in-act, but it can potentially be melted down and become a puddle, or be painted to be a different colour, such as pink, or be smelted into a ring, etc. All these things exist as being-in-potency alongside the being-in-act. This being-in-potency is a real feature of the thing in question and accords with its nature. Hence, it is within the nature of a gold bar to potentially be melted down and be a puddle, but the bar does not have the potentiality to grow legs and do the can-can. While it might be conceived that this could happen in one possible world, Aristotle’s talk of act-potency is not what modern philosophers today think about when discussing possible worlds. What a thing could do in any possible world is not to be equated with her potentialities; rather, a thing’s potentiality is related to a thing’s nature - something we will discuss in a future post. 

Aristotle argued that potency of a thing gives rise to act when something external interacts with the actual part of that thing. So, the external fire interacting with the actual car gives rise to the potentiality of goo. The potentiality of a thing itself cannot give rise to itself, otherwise it would be inexplicable why it has arisen to act at this time and not at another time, and this is true of things composed of parts that appear to move themselves, for it is the parts moving other parts rather than one part moving itself.

Now, it might be asked what exactly is being-in-potency. It is clear what being-in-act is - it is the actual stuff that we can measure. One possible answer is that we cannot know exactly what being-in-potency is but nevertheless can deduce that it is a necessary feature of the world to explain change. Alternatively, we could appeal to the mind of God and argue an occasionalist position, which says that the potency of a thing is what God has determined He will cause a thing to become when acted upon by certain other things. So, one might say that God has determined in His mind that plastic toy cars turn to goo when interacted upon by fire.