This essay takes its point of departure from Mark Solms' contention that Freud's neurological thinking informs his work throughout, and that the RSE renders this more thoroughly than the SE. Starting from this contention I examine and compare the RSE and the SE on the question Freud's theorization of difference from his neurological writings onward. I pay special attention to the subtle distinction in the original German of Freud's uses of Unterschied (difference between) and Differenz (difference).