5.4731 Self-evidence, which Russell spoke so much about, can only become superfluous in logic by language itself preventing each logical mistake. – That logic is a priori consists in the fact that nothing illogical can be thought.

Cf. 5.1363.

Black (p. 274) cites Russell’s “Philosophical importance” p. 490 and p. 492 as good sources for his views on self-evidence.

So language does prevent every mistake of a certain kind. Such mistakes cannot be thought. And this is because we have not defined some of their terms. So they might seem to be thoughts, but they are not in fact so. And it is language, not logic per se, that shows this.