Why this matters
This axiom keeps the frame stable. If something could distinguish itself, it would break into contradiction. The diagonal must stay sealed so that the rest of the structure can move without folding in on itself.
Pair requirement
Distinction always means a relation between two relata. 👁A and 👁B make a pair. With only one relatum, there is no between for the relation to name.
Contrast and distinction
Contrast is simpler. A single thing can stand against absence, presence against void. That is contrast. Distinction goes further. It works only when two relata stand apart.
Consequences for the circuit
On the diagonal there is no condition to test, no licence to grant, no return to form. The circuit begins only when two relata appear. Reasoning starts off the diagonal.
Punchline
Nothing distinguishes itself from itself. Distinction lives where 👁A and 👁B stand apart. Identity holds where they coincide.