Freud has a direct rebuttal to Wittgenstein's criticism that his theory of the unconscious is speculation, not science, because it is not subject to verifiability. Freud writes:
"Only quite a short while ago the medical faculty in an American University refused to allow psycho-analysis the status of a science, on the ground that it did not admit of any experimental proof. ... Nevertheless, some Viennese investigators have actually made a beginning with experimental confirmation of our dream symbolism."
Freud goes on to detail several experimenters who have found evidence of dream symbolism that is supposed to help confirm the existence of the unconscious.
Points of Contact and Criticism Between Wittgenstein and Freud