You can’t act as if you can’t act, for example. So, it is not true that you can’t act. Likewise, you can’t think that you can’t think; can’t be aware that you can’t be aware; can’t mean that there is no meaning; can’t yourself suffer the illusion that your self is an illusion; and so forth.
This is the practical aspect of the fundamental epistemological criterion of truth, which is adequacy to quotidian experience.
Extending this notion a bit further: you can’t say that there is no such thing as metaphysical truth other than by asserting a putative metaphysical truth. Ditto for moral truths, and aesthetic truths: you can’t say that morals or aesthetics are relative except by asserting a moral or aesthetic absolute. Indeed, this holds for any sort of truth. You can’t say there is no political truth other than by asserting a putative political truth, for example.
Nominalism and positivism both fall before this scythe. Nominalism can be asserted only by means of the very universals it reprehends. Positivism itself is among the propositional systems that cannot be logically or empirically demonstrated, and insists are therefore meaningless; so that its assertion is its contradiction.
Also, of course, you can’t for very long successfully live as if an important falsehood were true. We’ve all proved this for ourselves a million times.
Thus the very rejection of God is an implicit recognition of him. You can’t rebel against a nonexistent Lord.
The Orthosphere 
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Also known as “performative contradiction”
Aye. And there is a corollary: What cannot but be carried into practice must perforce be veridical.
To be fair, the more farsighted nominalists understand the paradox of their position.
Tractatus 6.54 “My propositions are elucidatory in this way: he who understands me finally recognizes them as senseless, when he has climbed out through them, on them, over them. (He must so to speak throw away the ladder after he has climbed up on it.) He must surmount these propositions; then he sees the world rightly.”
The truth is inexpressible, they will say, before turning the accusation around and saying that the rest of us mistake grammatical necessity for metaphysical necessity. They try to overcome metaphysics, and they end up mystics.
That’s pretty funny, in a mordant way. “You need to get over seeing the world the way I do in order to see it rightly.”
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Wait a minute. Isn’t your post positivism?
Hah! Good question. No. Positivism would say that propositions we cannot verify scientifically have no meaning. A statement that has no meaning can’t have truth value. On positivism, positivism is meaningless.
There are any number of statements that are logically demonstrable but that we cannot carry into practice in our world. They are meaningful, and can even be true, but they can’t be true of or about our world. So it is impossible for us to enact them; to behave as if they were true. It is impossible therefore for us to verify them scientifically.
I probably should have stated the notion more carefully: a proposition that cannot be carried into practice in our world can’t be veridical in our world.
If you can carry a proposition into practice, then, it is veridical. If you can’t, then even though it might be both meaningful and possibly true in some possible world, it isn’t true in ours.
The meaning may be of personal consequence, even if the statement has no way to be proven true or false. I think A. J. Ayers said that.
That’s Pragmatism’s definition of meaning: the meaning of a thing consists in its consequences in experience (positivism is the bastard of a misconstruction of Pragmatism, so this should not be surprising). I should say rather that the meaning of a thing consists in the qualia of its experience, but that amounts to the same thing.
That is a far, far wider net for meaning than positivism is generally willing to throw. It is, in fact, precisely, the net that we do all throw, at every turn. It admits to the tent of the meaningful virtually everything that positivism would like to keep out. It is the zero of positivism.
So I suppose that, like Hume, Ayer was provoked by the inescapable exigencies of life as lived to a massive unprincipled exception. Par for all erroneous courses.
Yes… “Radical autonomy” is false, but desire for “radical autonomy” is very true.
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Submission to temptation is a subversion of Truth. “It” is true insofar at it is truly false.