5.515 It must be manifest in our symbols that it can only be propositions
that are combined with one another by 'C', '.', etc. And this is indeed the
case, since the symbol in 'p' and 'q' itself presupposes 'C', 'P', etc. If
the sign 'p' in 'p C q' does not stand for a complex sign, then it cannot
have sense by itself: but in that case the signs 'p C p', 'p . p', etc.,
which have the same sense as p, must also lack sense. But if 'p C p' has no
sense, then 'p C q' cannot have a sense either.
- -- Wittgenstein, Ludwig