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By addressing the issues of psychogenesis, the mind/body problem, etc. psychoanalysis necessarily raises ontological questions.
The questions of whether Freud's views can be considered materialistic or not is difficult to answer.
On the one hand, he insisted on the importance of the physical substratum of all mental events, in keeping with the materialist axioms of the scientists whom he had most respected during his studies (principally Hermann Helmholtz and Ernst Brücke). On the other hand, he opposed Charcot's attempts to explain all hysterical symptoms by reference to lesions in the brain, distinguished psychical reality from material reality, and constantly emphasized the role of experience rather than heredity in the aetiology of nervous illness.
These two trends often converge in his writings in an uneasy alliance, as in the following sentence:
Analysts are at bottom incorrigible mechanists and materialists, even though they seek to avoid robbing the mind and spirit of their still unrecognized characteristics.
Lacan too presents himself as a materialist; in 1936 he criticizes associationist psychology for not living up to its purported materialism, and in 1964 he argues that psychoanalysis is opposed to any form of philosophical idealism.
However, as with Freud, Lacan's declarations of materialism are highly complex. It is clear in Lacan's earliest statements on the subject that he conceives of materialism in a very particular way. In 1936, for example, he argues that materialism does not imply a rejection of the categories of intentionality and meaning,  and he rejects the simplistic idea of 'matter' as "a naive form which has been left behind by authentic materialism."
In 1946 he repeatedly criticizes the crude form of materialism which regards thought as a mere "epiphenomenon." And in 1956 he distinguishes between a "naturalist materialism" and a "Freudian materialism".
It is clear, then, that Lacan does not subscribe to a kind of materialism which reduces all causation to a crude economic determinism which regards all cultural phenomena (including language) as a mere superstructure," and argues that language "is something material."
"the point of view I am trying to maintain before you involves a certain materialism of the elements in question, in the sense that the signifiers are well and truly embodied, and materialized."
However, the materiality of the signifier does not refer to a tangible inscription but to its indivisibility:
"But if we have insisted firstly on the materiality of the signifier, this materiality is singular in many ways, the first of which is that the signifier does not withstand partition."
- Freud, Sigmund. 1941d. (1921). SE XVIII p.179
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p. 76-8
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p.90
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p.159
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p.465-6
- Lacan, Jacques. Écrits: A Selection. Trans. Alan Sheridan. London: Tavistock Publications, 1977. p.125
- Lacan, Jacques. The Seminar. Book II. The Ego in Freud's Theory and in the Technique of Psychoanalysis, 1954-55. Trans. Sylvana Tomaselli. New York: Nortion; Cambridge: Cambridge Unviersity Press, 1988. p.82
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p.875-6
- Lacan, Jacques. The Seminar. Book III. The Psychoses, 1955-56. Trans. Russell Grigg. London: Routledge, 1993. p.289
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p.24
- Lacan, Jacques. The Seminar. Book II. The Ego in Freud's Theory and in the Technique of Psychoanalysis, 1954-55. Trans. Sylvana Tomaselli. New York: Nortion; Cambridge: Cambridge Unviersity Press, 1988. p. 40-52
- Derrida. 1975.