Some Fitch-style arguments

Let strong physicalist supervenience (SPS) be the view that necessarily all truths supervene on first-order truth about the spatiotemporal arrangement of physical stuff.

Here is a plausible thesis:

1. Every first-order truth about the spatiotemporal arrangement of physical stuff is possibly known.

Add this premise:

2. If a conjunction is known, so are its conjuncts.

It follows from SPS, 1 and 2 that:

3. Every actual first-order truth about the spatiotemporal arrangement of physical stuff is actually known.

For suppose that p is any actual first-order truth about the spatiotemporal arrangement of physical stuff. Let q be the conjunction of all other actual first-order truths about the spatiotemporal arrangement of physical stuff. Let r be the conjunction of p and q. Then r is a first-order truth about the spatiotemporal arrangement of physical stuff. By 1, there is a possible world w where r is known. Since r contains all actual first-order truths about the spatiotemporal arrangement of physical stuff, w and the actual world have the same first-order truths about the spatiotemporal arrangement of physical stuff. By SPS, it follows that w and the actual world have the same truths. Thus, since it’s true at w that r is known, it’s actually true that r is known. And since r is actually known, so is p by 2. Hence, we have 3.

In exactly the same way, given:

4. Every first-order truth about the spatiotemporal arrangement of physical stuff is possibly explained

5. If a conjunction is explained, so are its conjuncts

and SPS, we get:

6. Every actual first-order truth about the spatiotemporal arrangement of physical stuff actually has an explanation.

But given physicalism, it is clear that 3 and 6 are false. Regarding 3, it is clear that if physicalism is true, nobody knowns how many hairs I have on my head. Regarding 6, given physicalism, there is no explanation of the sum total of physical reality. Thus:

7. Given either 1 and 2, or 4 and 5, at least one of physicalism and SPS is false.

It is tempting to turn this into an argument against physicalism by holding that:

8. If SPS is true, physicalism is true.

But actually I can imagine someone denying 8. Imagine a classical theist who thinks that everything other than God has to be physical. Such a classical theist will actually believe SPS. For on classical theism, facts about God supervene on facts about everything other than God, since classical theism includes divine simplicity according to which there are no contingent facts intrinsically about God, so any two worlds that differ in their facts about God will have to differ in something other than God. And according to our classical theist, everything other than God has to be physical. So any two worlds that differ must differ in facts about the arrangement of physical stuff, which is what SPS says. However, the classical theist is not a physicalist, because God is not physical on classical theism.

Still, I think an argument against the conjunction of physicalism and SPS is still interesting.

An odd argument for an omniscient being

Here’s a funny logically valid argument:

1. The analytic/synthetic distinction between truths is the same as the a priori / a posteriori distinction.

2. The analytic/synthetic distinction between truths makes sense.

3. If 1 and 2, then every truth is knowable.

4. So, every truth is knowable. (1–3)

5. If every truth is knowable, then every truth is known.

6. So, every truth is known. (4–5)

7. If every truth is known, there is an omniscient being.

8. So, there is an omniscient being. (6–7)

I won’t argue for 1 and 2: those are big-picture substantive philosophical questions. I am sceptical of both claims.

The argument for 3 is this. If the analytic/synthetic distinction makes sense, then the two concepts are exclusive and exhaustive among truths: a truth is synthetic just in case it’s not analytic. So, every truth is analytic or synthetic. But if 1 is true and the analytic/synthetic distinction makes sense, then it follows that every truth is a priori or a posteriori. But these phrases are short for a priori knowable and a posteriori knowable. Thus, if 1 is true and the analytic/synthetic distinction makes sense, then every truth is knowable.

The argument for 5 is the famous knowability paradox: If p is an unknown truth, then that p is an unknown truth is a truth that cannot be known (for if someone know that p is an unknown truth, then they would thereby know that p is a truth, and then it wouldn’t be an unknown truth, and no one can’t know what isn’t so).

One argument for 7 is an Ockham’s Razor argument: it is more plausible to think there is one being that knows all things than that the knowledge is scattered among many. A sketch of a deductive argument for 5 that skirts over some important technical issues is this: if you know a conjunction, you know all the conjuncts; let p be the conjunction of all truths; if every truth is known, then p is known; and someone who knows p knows all.

A being that knows much

Here is an argument inspired by the Gale-Pruss cosmological argument.

1. (Premise) Every first-order truth is knowable.
2. (Premise) The conjunction of all basic first-order truths exists and is a first-order truth.
3. (Premise) If all the basic first-order truths of a world w₁ hold at a world w₂, then w₂=w₁.
4. Let p be the conjunction of all basic first-order truths. Let w be a world where there is a being, x, that knows p. (By 1 and 2)
5. (Premise) Necessarily, if someone knows p, then p is true.
6. p is true at w. (4 and 5)
7. w is the actual world. (3, 4 and 6)
8. There actually is a being that knows the conjunction of all basic first-order truths. (4 and 7)

Note that the restriction to first-order truths makes this not be just the standard knowability paradox.

I am not giving a definition of basicness. The crucial constraint is that the notion be such that both (2) and (3) hold. If one says that every first-order truth counts as basic, then (3) is very plausible, but (2) is less clear because one worries about a conjunction that has itself as a conjunct. Or maybe we could say that the basic first-order truths are truths expressed by literals, namely truths of the form of the proposition that P(a₁,...,a_n) for some predicate P or its negation. This requires a free logic, an existence predicate, and propositions about all possible non-existent entities. Alternately, we might take "basic" to just mean "fundamental".

Note added later: I forgot the word "basic" in (3) in the original argument.