[lisper]

> The earth is round, regardless of whether people have this idea in their heads or not. You deny this?

No. What I deny is that "the earth is round" and "the proposition 'the earth is round' is true" are identical propositions. The first is a proposition that makes a claim about the roundness of the earth, and the second is a proposition that makes a claim about the truth of a proposition. Those aren't the same thing.

The earth is round whether or not anyone is around to contemplate it (the earth). But "the earth is round" is not true unless someone is around to contemplate it (the proposition).

> What did Turing do that proved that propositions only exist insofar as they are conceived?

That's a long story, and I have to run to a meeting right now. If you really want to know, remind me later. I may need to write a blog post about it.

But forget about propositions for the moment. Do you think that unwritten poems exist?

[knightoffaith]

So, I think if saying "the earth is round" is true and saying the earth is round mean different things, then we haven't construed the former properly, the former should be construed as expressing the same thing as the latter. If it's just a linguistic disagreement, then I think we can set that aside, I'm not very interested in that. The original point was that we can intuit certain things as being true, e.g. that sense-data reflects something about reality, and there's no need to appeal to how useful believing this is or isn't. Like, conceivably, nuclear wars could lead to the annihilation of humanity, but I don't see why that should have any bearing on whether atomic bombs exist or not.

[lisper]

> the former should be construed as expressing the same thing as the latter. If it's just a linguistic disagreement, then I think we can set that aside

No, this is not just a linguistic disagreement. This is the crux of the matter. By saying that "the earth is round" and "the proposition 'the earth is round' is true" mean the same thing you are conflating two different ontological categories. When you do that, your reasoning is no longer sound.

Consider this:

P1: The U.S.S. Enterprise can travel faster than light.

Is P1 true? If yes, then how can that be when we know from relativity theory that nothing can travel faster than light? And if no, then what about these:

P2: The U.S.S. Enterprise is powered by a matter-anti-matter reaction controlled by dilithium crystals.

P3: The U.S.S. Enterprise is powered by squirrels running on treadmills.

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P.S. It occurred to me that there is a TL;DR answer to the question of what Turing did that was so important: he invented general-purpose software.