If everything is contingent in the relevant sense—i.e. if, for every particular thing, there is some time at which that thing fails to exist—then it surely is possible that, at some time, nothing exists. There doesn’t seem to be any argument or extra step needed here (why would one think it’s impossible that these things all fail to exist at the same time?). And once you have this possibility established, the Principle of Plenitude will get you its actuality. So again, I think the charge of the quantifier shift fallacy is off target. Perhaps I’m just misunderstanding your objection?

As to your second point, I think one really has to keep the Summa’s intended audience in mind. There are lots of things that St. Thomas can reasonably take for granted when he’s writing a textbook for Dominican seminarians. The Avicennian context of the 3rd Way is one of those things, I think.

Imagine a contemporary philosopher who pledges allegiance to the Credo of the Canberra Planners writing a paper on ontology, and framing arguments that take four-dimensionalism for granted. If the paper is a contribution to an ongoing debate among people who all take four-dimensionalism for granted, then—even if the person didn’t bother to flag that presupposition in a footnote—it might be fine to just take for granted what everyone else is taking for granted. Or imagine a paper on reducing modality that never bothers to try to convince us that reducing modality is a sensible project before giving its attempt to reduce modality. I myself am an antireductionist about modality and a presentist. So I would say that in both cases, dubious assumptions are being tacitly made. But I wouldn’t suggest that the arguments are thereby fallacious, or just as bad as fallacious. I would say, perhaps, that they are uncompelling (to me), because they rely on dubious assumptions. But, of course, the people making the assumptions won’t see them as dubious at all. Nor, perhaps will many of their readers.

Whether something is dubious is, at least in many cases, a person-relative question. Whether something is fallacious isn’t. And whether it’s legitimate to make certain assumptions is a context-relative question. Whether something is fallacious isn’t. So I’m strongly inclined to think that “The 3rd Way is uncompelling because it assumes the Principle of Plenitude” is a perfectly appropriate objection. And I’m strongly inclined to think “The 3rd Way is fallacious because it commits modal fallacies (or something very much like modal fallacies)” is not.

First, even with the Principle the argument doesn’t work. We still need a step that takes us from “Everything is such that it might not exist” to “It might have been that everything didn’t exist”. That’s not a logically valid argument, and indeed on some nominalist theories it will even have a true premise and a false conclusion.

Second, I don’t think that it’s much of a defence to a claim that X committed a logical fallacy by saying that with an extra premise the argument works. Every fallacious argument can be fixed up with an extra premise. To be fair, there could be good reason to think that Aquinas was taking this premise for granted, but I don’t think there’s much of a distinction between tacitly making dubious assumptions and committing logical fallacies.

The basic ideas I have in mind are that while [a] St. Thomas thinks it is necessary to experience certain very particular things in order to know that God exists, [b] his conception of a posteriority (“from what is prior relatively only to us”) is nevertheless importantly different from Kant’s (“borrowed solely from experience”), and so© I’m not sure that he thinks the existence of God is known a posteriori in Kant’s sense.

But this still leaves open the question of where Kripke fits on the map. I haven’t yet memorized Naming and Necessity, so I’ll leave this one up to others. In any case, though, while it does seem clear to me that Aquinas would regard “Water is H20” as a posteriori and necessary, I’m not sure that Kant would need to allow the former (cf. again Fodor).

Here’s the text: A thing can be self-evident in either of two ways: on the one hand, self-evident in itself, though not to us; on the other, self-evident in itself, and to us. A proposition is self-evident because the predicate is included in the essence of the subject, as “Man is an animal,” for animal is contained in the essence of man. If, therefore the essence of the predicate and subject be known to all, the proposition will be self-evident to all; as is clear with regard to the first principles of demonstration, the terms of which are common things that no one is ignorant of, such as being and non-being, whole and part, and such like. If, however, there are some to whom the essence of the predicate and subject is unknown, the proposition will be self-evident in itself, but not to those who do not know the meaning of the predicate and subject of the proposition. Therefore, it happens, as Boethius says (Hebdom., the title of which is: “Whether all that is, is good”), “that there are some mental concepts self-evident only to the learned, as that incorporeal substances are not in space.” Therefore I say that this proposition, “God exists,” of itself is self-evident, for the predicate is the same as the subject, because God is His own existence as will be hereafter shown (3, 4). Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature—namely, by effects.

If everything is contingent in the Aristotelian sense ironed out above, then it’s a real possibility that nothing exist. So if the Principle of Plenitude is true (and if the world is “eternal,” which this argument’s first advocate, Avicenna, would certainly have built into it) then at some time, there would be nothing real. There is no fallacy to be found in this line of reasoning.

That’s not to say there’s nothing objectionable about it. You might think the Principle of Plenitude is obviously false. Or you might think it’s obviously false that the world is eternal. But to say that an argument has an (arguably) false premise (or two) is quite different from saying that it commits a fairly elementary logical fallacy.

There’s the further concern, in interpreting the argument this way, that St. Thomas himself rejects the Principle of Plenitude, as well as the thesis that the world is eternal. So one might wonder why St. Thomas is advancing an argument that has premises he deems false. But, again, that’s a very different kind of worry.

Having said all that, I’m basing this on a fairly flat footed reading of the relevant passages, and someone who knows more of the context might be able to easily correct me.

1. First off, it does seem right to point out that philosophers prior to Hume (say) did believe that there could be necessary a posteriori truths, if by this we mean that there are necessary truths that we can come to know only by experience.

2. But it’s also not clear that these necessary a posteriori truths are of the sort whose possibility Kant dismisses in the first Critique and elsewhere; i.e. they require experience for us to come to know them, but we don’t justify them by way of the experience itself in the same way that we justify, e.g., the claim that there’s a computer on the desk. (I’m not sure if this is right, but it seems plausible to me.)

3. This, though, raises the question of whether Kripkean identities are a posteriori in Kant’s sense as opposed to (say) Aquinas’s or Aristotle’s. This is a tricky point. But one reason to suspect that they’re not is that merely pointing to the fact that the clear stuff in this glass is composed of H20 won’t show us that all water is necessarily composed of H20; something extra is needed. (I think Fodor makes a similar point in his LRB article “Water’s Water Everywhere”.) In any case, the way we justify “this particular stuff is made of H20” is quite different from the way we justify “this kind of stuff is made of H20”. Whether this shows that Kripke and Kant are on entirely different wavelengths, I don’t know. But again, it seems worth considering.

The upshot, I take it, is that it is (once again) important to get the history right on its own terms.

http://www.antimeta.org/blog/archives/2005/10/discovery_in_ma.html

The Soames link is definitely working, but it’s a Word doc and some computers may not handle that.

I agree entirely that being the first to know is not necessary for discovery. What I was really surprised to discover was that it’s not sufficient either. But I should have made this clear.

As David says, the big fallacy in the Third Way is going from “Everything might not exist” to “It might be that everything doesn’t exist”. Even if you read Aquinas as talking about temporal necessity or some such, so the ‘Aristotelian’ step isn’t a fallacy, there’s still that fallacy.

I agree too that there are plenty of ways other than being a Thomist to believe that God’s existence is necessary a posteriori. Presumably those who believe the argument from religous experience to be sound (and believe in God’s necessity) believe that God’s existence is necessary a posteriori. So I was wrong to suggest that it’s only Thomists who should be moved here.

Matt’s right of course that Kripke didn’t discover the necessary a posteriori truths – the scientists did. What he discovered was that they are necessary a posteriori. My bad.

Finally, as to why I’m interested in this. Soames in his reply lists three characteristics of the ordinary language philosophers. I happen to think Ryle, Strawson and Austin satisfied at most one of them (the same number that Quine and Williamson satisfy) so the list of characteristics probably isn’t right. But I think he left out one crucial and quite positive feature of the ordinary language crew. They thought it was important to look at lots of examples, not just the same old same old that we always look at (the bent stick, Macbeth’s dagger) and they thought it was important to learn about lots of relevant concepts, not just the ones we ordinarily consider (e.g. knowledge, belief, etc.) Rather than spending 26 weeks on KNOWLEDGE and none on DISCOVERY or LEARNING or the like, we should have a more even distribution. So I wanted to do some cheap conceptual analysis of ‘discovered that’, partially for fun, partially for the sake of the workout (this of this like the philosophy gym) and partially because it might reflect something back about knowledge.