If physicalism is true, then the nature of human beings is probably essentially tied to the nature of our brains, which in turn is essentially tied to the laws of nature. So a human being couldn't have a radically transformed brain. But there are limits on the information storage of our brains. This makes plausible the first premise of the following valid argument:
- If physicalism is true, there are only finitely many integers that it is metaphysically possible for humans to think about (without externalist crutches).
- It is metaphysically possible for humans to think about any integer (without externalist crutches).
- So, physicalism is not true.