In last week’s post, I argued that the study of modality is important because claims about possibility, impossibility, and necessity show up in various disciplines including ethics, logic, physics, theology, and metaphysics. Modal epistemology is concerned with how we might know such claims to be true, and modal ontology explores that in virtue of which modal propositions are true. In this week’s post, I will briefly review the first of eight theories of the nature of possibility, as described in Alexander Pruss’ book, Actuality, Possibility, and Worlds (2011). I call this first theory, “The Necessity View.”
According to The Necessity View (henceforth, TNV), only the actual world is possible. Whatever is actual must be actual, and there is no other way it could have been. There are no alternative possibilities, and therefore no possible worlds. Everything that exists has to exist, and in the precise way it does. But what explains the necessity of the actual world? The answer is found in a rational or ethical principle that explains why everything else must be. I will explore two such principles: the Principle of Sufficient Reason (PSR) and the Axiarchic Principle (AP).
THE PRINCIPLE OF SUFFICIENT REASON (PSR)
One version of the PSR states that every truth has an explanation that entails it. Philosophers like Parmenides and Benedict de Spinoza used that principle (or something close to it) to argue that there are no contingent truths, only necessary ones. Their argument can be stated as follows.
- Every true proposition has an explanation that entails its truth (PSR).
- True propositions are either necessary or contingent.
- If some propositions are contingently true, then there is a conjunct C of all those propositions.
- Suppose, for the sake of argument, that there is conjunct C of all contingently true propositions.
- If so, then the explanation of C’s truth is either necessary or contingent.
- The explanation of C cannot be contingent because then it would be part of C, leaving C without an explanation.
- The explanation of C cannot be necessary because it holds true in possible worlds where C fails to obtain – i.e. it doesn’t entail C.
- Therefore, the explanation of C is neither necessary nor contingent (from 6 & 7).
- Therefore, there is no conjunct C of all contingently true propositions (4, 5 & 7).
- Therefore, there are no contingent truths (from 3 & 9).
- Therefore, only necessary propositions can be true (from 2 & 10).
- Ergo, TNV is true.
The argument that only necessary propositions are true can be challenged on several fronts. First, if the PSR is applied without qualification, then it leads to an infinite regress of explanations; so unless this regress is benign, the PSR will need to be rejected. Second, the argument leads to counterintuitive results. If everything is necessary, then so are my choices, and this is hard to square with my being a free and responsible moral agent. Also, I have a very strong modal intuition that some things might not have been. Propositions like “Hitler did not exist” or “Obama lost the presidential election” are false, but they are not necessarily false. At the very least we need compelling reasons to override this intuition. Third, a necessary truth need not entail a proposition in order to explain it. All it needs to do is raise the probability of that proposition, all other things being equal. For example, if God is a broadly logically necessary being, then his existence raises the probability that a contingent universe exists by virtue of his causal powers and his reasons. His powers and reasons exist in every possible world, but they do not entail that a physical universe exists. Why? Because God’s free actions are not determined by his reasons. So contrary to premises (1) and (7), a necessary proposition like “God exists” can explain a contingent proposition like, “a universe exists.” Fourth, Alex Pruss (1996) suggests that some contingent truths are self-explaining, but I will not delve into that claim here (p.27).
THE AXIARCHIC PRINCIPLE (AP)
Axiology is a field of study that explores the nature of value – what kinds of values there are (e.g. beauty, truth, goodness, optimality, etc), whether they are objective or subjective, how they are weighed or compared, and their relation to duties.
One particular theory of value championed by John Leslie is known as “axiarchism.” This theory holds that values rule or explain the natural order. Values (such as the Good) are objectively real and have creative power. Though the Good exists in an abstract realm apart from concrete particulars, it necessarily generates every particular thing that should exist. Admittedly, the idea that something exists just because it ought to exist seems rather bizarre at first, especially because persons are normally understood to be the causes of what should be. But without the assumption that only concrete entities have causal powers, the view that the Good necessarily generates every particular thing that ought to exist begins to make more sense. The activity of the Good can be formulated according the following axiarchic principle (or AP): “for every concretely realizable property p, if it ought to be the case that some concrete thing realizes p, then there exists some concrete thing that does realize p” (Steinhart, 2013), p.1). 
Now if AP is true, then the only concrete things that can exist are the ones that actually realize every property that is ethically required. If so, then only the actual world can exist because different worlds would be a deficient expression of the Good
What can be said about the drawbacks of AP? At first glance, it is unclear why only one world is required by AP. If several alternatives meet the same ethical requirements, then it is not necessary that one world should exist rather than another. It is also unclear that any world is required by AP, because worlds can always be improved. AP cannot prescribe one world as the best if there is no upper limit to what is best. Thus, if several worlds are consistent with AP, then Axiarchism is not really a version of the Necessity View after all. Third, the AP is difficult to reconcile with the problem of evil because the kind, amount, intensity and duration of apparently gratuitous suffering in the world weighs heavily against the radical claim that everything is for the best. Perhaps this difficulty can be mitigated if some of the goods required by AP have suffering as their precondition or side-effect.
What do you, the reader, think about the arguments from PSR and AP? Do you find the Necessity View convincing? Why or why not? Are there other arguments that deserve attention?
 Alex Pruss believes that a viable ontology of modality will “need to explain what the truthmakers of modal propositions are, and what it is about these truthmakers that makes them suitable to be such” (p.20).
 This 12 step argument was inspired by Pruss (1996), p. 26.
 See Pruss (1996) Possible Worlds: What They Are Good for and What They Are. University of Pittsburgh.