One of the reasons Wittgenstein thought the exclusive use of ostensive definition failed was because it opened the possibility that a given symbol's meaning was sourced from whatever sensation someone was having at the time, so that a symbol's meaning was whatever seemed to be the case to the private linguist.
He then concluded that in this situation truth and falsity couldn't be rationally talked about. This point was made because Wittgenstein thought that the values of truth and falsity could only apply to cases where the one or the other could actually be applied.
In other words, Wittgenstein thought that only symbols that could be doubted could have meaning and the values 'truth' and 'falsity' applied to them.
But why is this?
It seems to be the case that Wittgenstein thought that otherwise what was being dealt with was a tautology, where the sense of meaning was lost.
But couldn't there be other meaningful self-evident sort of statements where what's occurring isn't a tautology and where it wouldn't be reasonable nor conceivable to doubt?