I. Parmenides and the Problem of Change
In pre-Socratic Greek philosophy, Parmenides reasoned that change (like rivers running, wind blowing, sun burning) does not occur. He divided the world into two: things either exist, or they don't. Anything that is already is, and anything that does not exist cannot cause anything. So change does not occur in spite of what our senses tell us.
II. The Solution to Parmenides - Aristotle
Aristotle solved the problem by further dividing what exists into actual and potential. If something is actual, then it is real. An actual block of ice, for example. However, that thing could potentially be different. It is potentially on the floor, potentially on the table, potentially broken in two, potentially a puddle of liquid, and so on. So when we speak of change, we can say that something's potential became actual.
III. Two Different Causal Series
Consider a chain of causes: A causes B, and then B causes C. This chain stretches "back" in time and could in theory have been going for an infinite amount of time. For example, a chicken laying an egg, which then hatches and becomes a new chicken, which lays an egg, and so on. This is called an accidentally ordered series. Since the past is an example of an accidentally ordered series (the past doesn't need to be around anymore to make the present work), it is for this reason that Aquinas didn't think it could be proven that the universe had a beginning.
Now consider a chain of causes, where A directly causes C, but it is passed through intermediate member B, which can only "pass it along" from A. A is the only one doing any work. B has no power to cause C. For example, consider a hand moving a stick moving a rock. The stick can only pass along the motion from the hand to the rock; it can't move the rock itself. This chain stretches "down" in the present. It can't be an infinitely long chain, because then there would in effect be no A and nothing would be happening to C (if the chain of sticks were infinitely long, then there would be no hand, which is the only one doing any work). This is called an essentially ordered series.
IV. Premise 1 of the First Way: Some things are changing
Things around us are having their potentialities actualized. The "actual ice/potential liquid" melts and becomes "actual liquid/potential ice."
The first premise is: some things are changing.
V. Premise 2 of the First Way: Whatever is changing is being changed by something else
A potential is merely a potential, and not actual. A potential ice cube cannot make your drink frosty cold. Since a potential is not actual, it cannot make itself actual.
So it must be made actual by something else that is actual. The actual cold air in the freezer, for example, is needed to make the potential ice (actual liquid) into actual ice. If that thing is having its potential actualized too, then it must also be changed by something else that is actual. For example, actual electricity must make the potentially running freezer actually run. And so on in a chain.
So the second premise is: whatever is changing (whatever is a mix of potential & actual) is being changed by something else.
What About Quantum Mechanics?
Doesn't quantum mechanics show that a potential can make itself actual, such as when an atom undergoes decay, or when virtual particles pop into existence? There are three reasons why this objection will not work:
- QM merely describes behavior; it does not explain the occurrence of that behavior.
- Causality includes electromagnetism, a billiard ball knocking into another one, sunlight causing plant's to grow, situations causing rational agents to act, and so on. Only if causality reduces down to a crude model of one billiard ball knocking into another one could QM provide some reason to think the principle of causality is false.
This chain cannot be infinitely long, because it is an essentially ordered series. There must be a member at the head that is actualizing the whole chain (remember the hand moving a stick moving a rock, or the laser example in the diagram above). In the example so far, it would be somewhere in the coal at the power plant.
In the example so far, it would be somewhere in the coal at the power plant:
That driving agent can be either A) just potential, B) a mix of potential and actual, or C) just actual.
(A) is logically incoherent; nothing can be just potential with no actual existence. If it were (B), then it would not be the required first member of the chain, because per premise 2 above it would require something else to change it. So, the first member must be (C): just actual, with no potentials at all.
VII. Attributes of Pure Actuality
These are the attributes of something that is purely actual, with no potentials:
- Matter/energy are changeable, and hence are a mixture of potential and actual. Therefore, an agent of pure actuality is incorporeal.
- Things composed of matter and form and actual and potential are varying levels of complexity, but an agent of pure actuality has no parts, no matter, and no form, and thus is extremely simple.
- The only way to tell the difference between two things is if one thing has an attribute that another lacks. With two different beings, one is to the left (say), and one is to the right. The one on the left is potentially to the right, and vice versa. Both are hence mixtures of potentiality and actuality. But an agent of pure actuality by definition has no potentialities, and thus lacks nothing, and therefore every agent of pure actuality is really the same one agent. There is only one agent of pure actuality, and thus it has unity.
- An agent of pure actuality cannot come into or go out of existence, as these are potentialities being realized. Thus, an agent of pure actuality is eternal.
- An agent of pure actuality cannot change from one state to another, and so it is immutable.
- Being the source of all change in the universe, and the source of any change that could ever logically occur, an agent of pure actuality is omnipotent.
- This one is a bit tricky and really requires an understanding of the entire Thomistic system to do it justice, but a purely actual agent is the source of all change and everything that could ever happen or will happen. As shown in the Second Way as well, the purely actual agent is the source of the continual existence of everything. Thus it is the grounding of every true proposition. Therefore, it doesn't have knowledge, but it is knowledge itself. As Aquinas would say, it is only intelligent in the analogical sense, like saying that love is blind and Ray Charles is blind, but not in the univocal sense, like saying that stop sign is red and the firetruck is red. So, in the analogical sense, a purely actual agent is omniscient.
- A being of pure actuality cannot be made to change, as that would meant that he was mixture of potential and actual. So the agent is also impassable.
- Thomism includes the view that evil is a privation of good. "Goodness" on this view does not mean just "a nice guy", but rather "goodness" is synonymous with being, or existence. The dog that is missing a leg is suffering from evil because he lacks something: his leg. But an agent of pure actuality, or existence itself, does not by definition lack anything and is therefore wholly good.