Either he did or he didn't. Suppose I say that he did, and you say that he didn't. We both presuppose, inter alia, that there was a man named 'Kepler.' Now that proposition that we both presuppose, although entailed both by Kepler died in misery and Kepler did not die in misery is no part of what I assert when I assert that Kepler died in misery.
Why not?
Well, to proceed by reductio, if what I assert when I assert that Kepler died in misery is that (there was a man named 'Kepler' & he died in misery), then what you assert when you contradict me is that (either there was no man named 'Kepler' or that he did not die in misery). But the latter is not what you assert, and the former is not what I assert. That is because we take it for granted that there was a man who rejoiced under the name 'Johannes Kepler.'
What I assert is that Kepler died in misery, and what you assert is that Kepler did not die in misery. But we both presuppose that there was a man named 'Kepler.' The proposition that we both presuppose, while entailed by what we each assert, is not part of what we each assert.
That, I take it, is Frege's famous argument in Ueber Sinn und Bedeutung.
It seems pretty good to me.
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