In a simplified sense I think @Banno is saying that he can stipulate anything he likes without being subject to questions about justification. He could stipulate “1=2” just as he could stipulate “1=1”, and he would not need to justify anything. Perhaps in the first case the three letters (‘1’, ‘=’, and ‘2’) are stipulated to mean something different than they do in the second case (‘1’, ‘=’, and ‘1’; where we could further stipulate whether the “same” letter on the right and left side have the same meaning).
But if you press these issues you are going to run into nominalism and voluntarism. The OP is presupposing a paradigm of realism and intellectualism, where truths can be known. In the nominalist/voluntarist paradigm everything gets reduced to will instead of intellect, there are no truths in the classical sense, and all language is reduced to mere words (“nomina”) divorced from reality.
The thoroughgoing voluntarist sees a discussion about the OP as being the same as a discussion about chess:
- Eudemian: Why did you move your bishop?
- Voluntarist: Because I wanted to.
- Eudemian: Why did you move him diagonally?
- Voluntarist: Because chess requires it.
- Eudemian: Why are you playing chess?
- Voluntarist: Because I wanted to.
On this view there is no truth to be had; there is no knowledge to be had. In the end there are only the different things we in fact do, and the only reason we do them is because “we wanted to” (will/voluntas). It doesn’t make much of a difference whether the answer is, “I wanted to,” or, “We/society wanted to.” It has nothing to do with that goulish spectre of “Foundationalism,” but rather with the deflationist’s insistence that there is no normativity, not even as relates to truth and falsity, justification, etc.
@Banno is always a step away from this move, and he will take it when pressed. Still, his tradition flows from empiricism in general and logical positivism in particular, and that tradition is surely not confessedly nominalistic. Banno retains realist streaks in his philosophy. @Jay, on the other hand, is strongly drawn to the nominalist and voluntarist paradigms, albeit for largely moral reasons. In any case, both of them will try to deny the terms of your OP.