Marx was, before all else, an economic historian and theorist. That is what he devoted most of his mature years to, and that is the subject of more than five thousand pages of his published and unpublished writings, including all three volumes of Capital, the three volumes of Theories of Surplus Value, the Grundrisse, and A Contribution to the Critique of Political Economy. Freud was a medical doctor, trained as a neurologist. He spent his entire professional life, six hours or more each day, seeing patients. His speculative writings, like Civilization and its Discontents, and his excursions into armchair analysis [of precisely the sort that his own theories said was impossible], such as Moses and Monotheism, are no more expressions of his most accomplished and professional work than The Holy Familyis an expression of Marx at his most serious.
In trying to understand great thinkers like Marx and Freud, I find it useful to remind myself what they actually spent most of their time doing. Both Marx and Freud have been hijacked by Literary Criticism -- I do not think that is too strong a word. Neither of them would recognize himself in what has been written by those who claim to be their followers or to have been inspired by them. Marcuse, despite giving very little evidence of familiarity with Economics, was, I think, actually being true to both thinkers in his efforts at a rapprochement between them.
Freud was, I repeat, trained as a neurologist, as a medical doctor. His theory of the unconscious was an attempt to interpret observations made in the course of his treatment of patients. To the end of his life, as his writings make clear, he assumed that there must be a neurological basis for everything he observed and theorized about, despite the fact that, as he well knew, medical science was not in his day advanced enough to provide more than the sketchiest anatomical and neurological grounding for what he called the Unconscious, the Libido, the Id, the Ego, the Superego, and so forth. My own guess is that were he to return now and discover what could be done with brain scans and MRI's and CT scans and the rest, he would be thrilled and delighted. The one thing he absolutely would not do is retreat into literary theory or ideological critique as a sanctuary protected from the latest advances of hard science.
One further point about Freud: the focus of his medical practice was the treatment of what he called "neuroses." He was well aware that there were many other psychological illnesses and presentations that did not fall into that category -- psychoses, psychopathologies, and so on. He did not think his theories were the key to treating all mental illness -- only neuroses. It is useful to keep that in mind.
