Whether a mathematical proposition is true or not is indeed independent of physics. But the proof of such a proposition is a matter of physics only. There is no such thing as abstractly proving something, just as there is no such thing as abstractly knowing something. Mathematical truth is absolutely necessary and transcendent, but all knowledge is generated by physical processes, and its scope and limitations are conditioned by the laws of nature.
Of course there is. A proof of a mathematical proposition is just as much itself a mathematical object as the proposition being proved; it exists just as independently of physics. The proof as written down is a physical object standing in the same relation to the real proof as the digit 2 before your eyes here bears to the real number 2.
But perhaps in the context Deutsch isn't making that confusion. What scope and limitations on mathematical knowledge, conditioned by the laws of nature, does he draw out from these considerations?
-1TimS
The Pythagorean theorem isn't proved or or even checked by measuring right triangles and noticing that a^2 + b^2 = c^2. Is the Pythagorean theorem not knowledge?
-David Deutsch, The Beginning of Infinity.