That's not what the Church-Turing thesis is about. The Church-Turing thesis says that any "effective procedure" (i.e. algorithm) for performing computations on

That's not what the Church-Turing thesis is about. The Church-Turing thesis says that any "effective procedure" (i.e. algorithm) for performing computations on natural numbers is equivalent to a Turing machine. It is generally accepted but cannot be proven because "effective procedure" is not defined. Indeed it can be seen as providing a definition for the term "effective procedure". To extend that to say anything about the physical world (i.e. some form of "Physical Church-Turing Thesis") is to make quite a different assertion and one that is not so "generally accepted". To make it say anything about brains you have to assert some form of equivalence between brains and "effective procedures" on the natural numbers, which is indeed to beg the question in this instance. There's a different idea you may be confusing it with, which is the idea that since the known laws of physics (aside from state vector reduction in quantum mechanics) are in the form of differential equations that can be numerically integrated to arbitrary precision anything can be "simulated". However, this is not part of the Church-Turing thesis. (And personally I find the phrase "to arbitrary precision" here, and the exception for state vector reduction, quite a barrier to basing any assertions about the nature of the universe or minds on it.)