The Third Way: Necessary Being

The Third Way is clearly building on the Second Way. It adds an important component to the conclusion: we reach the conclusion that the uncaused cause of things must be a necessary being, in contrast to the contingent beings that are familiar to us. This immediately raises the question of what Aquinas means by 'necessary'. There are a range of possible interpretations:
 

1. Logically necessary. Its non-existence “entails a contradiction.” Or, perhaps, its non-existence is not ideally conceivable. Or, perhaps, its existence is an “analytic” truth (true by virtue of the meanings of the terms involved).
2. Metaphysically necessary. The narrowest form of real necessity. Not relative to a system of logic, and not epistemically or semantically defined.
3. Naturally necessary. Necessary given the metaphysically contingent order of nature within creation. Its existence is necessitated by the existence of anything or any kind of thing found in the actual creation.
4. Actually existing at every time.
5. Naturally incorruptible.
   

         a. Actually exists at some time and cannot naturally be corrupted.
         b. Actually exists at all times and cannot naturally be corrupted.
 
We can set aside meaning 1, since the argument has nothing to do with logical deducibility, analyticity, or conceivability.
 
Anthony Kenny argues (in his book, The Five Ways) that the Third Way must intend ‘necessary being’ in sense 5a or 5b. This is based on Aquinas’s assertion in the Summa Contra Gentiles (Book II, chapter 30) that the heavenly spheres are “necessary beings,” despite being created by God. However, this evidence does not rule out 3 or 4 either, although it does rule out 2.
 
​The next point of complexity concerns the distinction between things that are necessary per se (in and through themselves) and those whose necessity is caused by another. Are each of the four relevant categories of necessity (2, 3, 5a, and 5b) divided into two sub-categories by this distinction, or can some forms of necessity exist only per se or only when caused by another?
 
Now, suppose that the distinction cuts across all four meanings. Suppose, further, that Aquinas intended ‘necessary’ to correspond to meaning 3, 5a, or 5b. If so, the only result that Aquinas could reach would be that God is an incorruptible being that exists at all times or with merely natural (created) necessity. God is certainly incorruptible, but it wouldn’t make sense to attribute to Him existence in and through time, or merely natural necessity.
 
So, we must assume either that Aquinas intended meaning 2 throughout, or that he intended ‘necessary’ to include things that are necessary in senses 2, 3, 5a, or 5b, but he believed that only things with necessity in sense 2 (metaphysical necessity) could be necessary per se. Given his affirmation of created, metaphysically contingent beings (like the heavenly spheres) as ‘necessary’ in SCG II.30, the second assumption seems most likely. So, we can assume that necessary beings include all beings that are either metaphysically or naturally necessary, and that only metaphysically necessary beings are necessary per se. This leaves open the question of whether there could be metaphysically necessary beings (meaning 2) whose necessity is caused by something else. The most plausible candidates would be the divine ideas, although Aquinas never says that the necessary being of the divine ideas is caused (Summa Th. I q15, a1 and a2). He suggests instead that each divine idea is in some sense “identical” to the divine essence, despite the existence of many ideas.
 
The safest interpretation might be that only God is metaphysically necessary and only God is necessary per se (so those two categories coincide). Consequently, all of the merely naturally necessary beings have their necessity caused by God.
 
We still have to consider meanings 5a and 5b, which were favored by Kenny. I would rule out 5a immediately, since Aquinas never, as far as I know, labels the human soul as ‘necessary’, even though it is certainly incorruptible. It is true that Aquinas states (in Summa Theologica I q75 a6 ad 2) that the human soul has no ‘potentiality for non-existence’ but it isn’t clear that that is the same as necessary being.
 
The heavenly spheres are necessary beings in both sense 3 and 5b. So, the acid test will be to see which meaning makes most sense of the premises of the first part of Thomas’s argument. I will turn to this problem next week.