Yes, that one.
Although I don't know why you say "description of any given mathematical statement" instead of just "any given mathematical statement."
It's because I'm an idiot. I should have said descriptive aspect.
But there's the description, and there's the thing being described. The latter is part of what is meant by statement. Here's a quick argument summarizing why this is true...
A
statement is, formally, a sentence
that has, associated with it, a definite truth value. If your sentence does not have a definite truth value (that is, it is either true, or it is false), then you do not have a statement.
The thing that gives statements truth values is the thing
meant by the statement, and that involves some sort of claim about something. That something is the thing being described.
Now, what you have with
this "isomorphism" is merely the description per se, along with just enough meaning to refer to something. All of that part of "there is an SCGC" is in my brain when I talk about the statement "there is an SCGC". But something is not in my brain about this. I know what I'm talking about, and I know what has to be the case for that statement to be true, or to be false, but I don't know if it
is true, or if it
is false. What's missing is that I don't have, in my brain, the logical chain that establishes whether or not there actually is an SCGC--the sequence of steps that goes from what I know to what I would like to know. This thing I don't have--the
logical extension of relationships (you can call a particular connected chain a proof)--is the subject of mathematics.
This subject of mathematics is what makes the statement that there exists an SCGC have a
definite truth value. It's not in my brain.
Where is it?
And that is all I am arguing. If there is a computational theory, then there is necessarily a physical system that behaves in a way isomorphic to that theory.
Okay, but there are things in math that are not even in my brain. But they are part of mathematical statements that I can make, right now, today. They are, specifically, the part that gives the sentence its definite truth value--the part that qualifies the sentence for the next stage of precision of labeling... as a "statement".
It is not outside the realm of possibility for every single one of these to actually be manifest somehow in physical reality, by a mapping. But such is not
necessarily true.
Going the other way--from all physical statements to mathematical statements--is much less problematic.