Friday 3 December 2021

The Threefold Root of the How-Question About Mathematical Knowledge

Platonism is the default, almost obviously correct view about mathematical objects. One of the major things that puts pressure on Platonism is the question 'How do we know about mathematical objects, then?'. What gives this question its power? I think three things conspire and that the third might be under-appreciated:

1. Real justificatory demands internal to mathematical discourse. For particular mathematical claims, there are very real 'How do we know?' questions, and they have substantive mathematical answers. The impulse to ask the question then gets generalized to mathematical knowledge in general, except that then there's no substantial answer.

2. A feeling of impossibility engendered by a causal theory of knowledge. If you only think about certain kinds of knowledge, it can seem plausible that, in general, the way we get to know about things is via their causal impacts on us. This then makes mathematical knowledge seem impossible.

3. Our deeply-ingrained habit of giving reasons. The social impulse to justify one's claims to another is hacked by a monster: the philosophical question at the heart of the epistemology of mathematics.

If it were just 1 and 2 getting tangled up with each other, the how-question would not be so persistent. With existing philosophical understanding we'd be able to see our way past it. But 3 hasn't been excavated yet and that keeps the whole thing going.

Saturday 20 November 2021

A Puzzle about Abbreviation and Self-Reference

Let us use 'ONE' as an abbreviation of 'This sentence token contains more than one word token'. Now consider whether the following is true:


Friday 5 November 2021

Major Inaccuracies in Misak's review of Journey to the Edge of Reason, a new Gödel Biography

In the Times Literary Supplement there is a review of a new biography of Gödel biography by philosopher Cheryl Misak. The review is called 'What are the limits of logic? How a groundbreaking logician lost control'. This paragraph contains two major inaccuracies:

Gödel proved that if a statement in first-order logic is well formed (that is to say, it follows the syntactic rules for the formal language correctly), then there is a formal proof of it. But his second doctorate, or Habilitation, published in 1931, showed that in any formal system that includes arithmetic, there will always be statements that are both true and unprovable. The answer to the Entscheidungsproblem was, therefore, negative.

The first one is that being well formed is like being grammatically correct - among the well formed formulas of first-order logic, there are formulas that are false no matter what (false on all models or interpretations), formulas that can go either way, and formulas that are true no matter what (these ones are often called logical truths, or logically valid formulas). What Gödel showed is that for every logical truth, there is a proof that it's a logical truth. 

The second major inaccuracy is that the answer to the decision problem (Entscheidungsproblem) is not shown to be negative by the incompleteness theorem that Misak alludes to. The negative answer became known only in 1936, when Alonzo Church and Alan Turing independently showed it.

Thursday 21 October 2021

Reversing Logical Nihilism - Forthcoming in Synthese

Final draft available on PhilPapers.

Abstract: Gillian Russell has recently proposed counterexamples to such elementary argument forms as Conjunction Introduction (e.g. ‘Snow is white. Grass is green. Therefore, snow is white and grass is green’) and Identity (e.g. ‘Snow is white. Therefore, snow is white’). These purported counterexamples involve expressions that are sensitive to linguistic context—for example, a sentence which is true when it appears alone but false when embedded in a larger sentence. If they are genuine counterexamples, it looks as though logical nihilism—the view that there are no valid argument forms—might be true. In this paper, I argue that the purported counterexamples are not genuine, on the grounds that they equivocate. Having defused the threat of logical nihilism, I argue that the kind of linguistic context sensitivity at work in Russell’s purported counterexamples, if taken seriously, far from leading to logical nihilism, reveals new, previously undreamt-of valid forms. By way of proof of concept I present a simple logic, Solo-Only Propositional Logic (SOPL), designed to capture some of them. Along the way, some interesting subtleties about the fallacy of equivocation are revealed.

Saturday 28 August 2021

Validity as (Material!) Truth-Preservation in Virtue of Form

Forthcoming in Analytic Philosophy and available on PhilPapers.


According to a standard story, part of what we have in mind when we say that an argument is valid is that it is necessarily truth preserving: if the premises are true, the conclusion must also be true. But—the story continues—that’s not enough, since ‘Roses are red, therefore roses are coloured’ for example, while it may be necessarily truth-preserving, is not so in virtue of form. Thus we arrive at a standard contemporary characterisation of validity: an argument is valid when it is NTP in virtue of form. Here I argue that we can and should drop the N; the resulting account is simpler, less problematic, and performs just as well with examples.

Sunday 6 June 2021

Dialetheism and Transition States

I've been thinking recently about an argument given by Graham Priest for the view that, when you're on your way out of a room, there's a point in time at which you're both in the room and not in the room. 

For those who believe in true contradictions (dialethias) already because of Liar-like phenomena, whether or not this kind of example also exist may not seem like such an urgent issue. But if, like me, you are drawn to a gappy rather than a glutty response to Liar-like paradoxes, this sort of argument becomes more important, since it may take you from not believing in true contradictions to believing in them.

Priest's argument appears on p. 415 of his 'What is So Bad About Contradictions?' and is discussed in Section 3.4. of the SEP article on dialetheism. From the latter:

Transition states: when I exit the room, I am inside the room at one time, and outside of it at another. Given the continuity of motion, there must be a precise instant in time, call it t, at which I leave the room. Am I inside the room or outside at time t? Four answers are available: (a) I am inside; (b) I am outside; (c) I am both; and (d) I am neither. There is a strong intuition that (a) and (b) are ruled out by symmetry considerations: choosing either would be completely arbitrary. (This intuition is not at all unique to dialetheists: see the article on boundaries in general.) As for (d): if I am neither inside not outside the room, then I am not inside and not-not inside; therefore, I am either inside and not inside (option (c)), or not inside and not-not inside (which follows from option (d)); in both cases, a dialetheic situation. Or so it has been argued. For a recent description of inconsistent boundaries using formal mereology, see Weber and Cotnoir 2015.

I will now outline what I think we should say in response. 

The appeal to 'symmetry considerations' is where the trouble is in this argument. Let us assume for a moment that there are no true contradictions and see what we can say that is consistent with that commonsense view. On pain of contradiction, the notion of being 'inside' a room is not both true of me and false of me in this case. Now, we can grant that any notion of being 'inside' which is true of me in this case is in a sense biased in favour of insideness, and any notion of being 'inside' which is false of me in this case is biased against insideness. But probably, our normal notion of 'inside' as applied to rooms is just not determinate here; our linguistic behaviour doesn't make it the case that we are using the inside-biased notion rather than the outside-biased notion, nor does it make the opposite the case. But if we need to, we can just pick one of these notions, and everything will be OK as long as this is understood by speakers and hearers.

To me, this seems highly plausible. And it lets us maintain that there are no true contradictions. Let's grant Priest that it's bad philosophy to just reject outright the idea that there might be true contradictions. But if we haven't already adopted dialetheism, which is a pretty radical view, then other things equal, we should look for less drastic ways to make sense of examples like this. And that's what I've sketched above.

Thursday 15 April 2021

Forthcoming in Philosophical Studies: A Simple Theory of Rigidity


After wrestling inconclusively with this topic a couple of years ago on this blog, I came to a much clearer view about rigidity, which I set forth in this paper:

A Simple Theory of Rigidity

The notion of rigidity looms large in philosophy of language, but is beset by difficulties. This paper proposes a simple theory of rigidity, according to which an expression has a world-relative semantic property rigidly when it has that property at, or with respect to, all worlds. Just as names, and certain descriptions like The square root of 4, rigidly designate their referents, so too are necessary truths rigidly true, and so too does cat rigidly have only animals in its extension. After spelling out the theory, I argue that it enables us to avoid the headaches that attend the misbegotten desire to have a simple rigid/non-rigid distinction that applies to expressions, giving us a simple solution to the problem of generalizing the notion of rigidity beyond singular terms.