As far as I know, Whitehead never read Wittgenstein. He told me, however, of an encounter with Wittgenstein which was entirely characteristic of the man and may interest you.The Whiteheads, at my suggestion, invited Wittgenstein for a social tea. Wittgenstein came and, as was his wont, began to silently pace back and forth across the room. Finally, he declared, "A proposition has two poles; they are apb." Naturally enough. Whitehead enquired, "What are a and b?" "They," replied Wittgenstein with some solemnity, "are indefinable."