First of all, the fact that we're having this interesting discussion on psychoanalysis and the nature of knowledge is just another example of why psychiatry is, to me, the most fascinating area of medicine.
Philosophy is a priori knowledge and Science is a posteriori.
I'm no epistemologist, but I do think that this discussion is grossly oversimplifying the nature of scientific discovery -- specifically in the area of medicine. Medical research utilizes both a priori and a posteriori reasoning to reach its discoveries.
Classically, we tend to think of a scientist as coming up with a hypothesis a priori and testing that hypothesis through experimentation to prove it a posteriori. For instance, based on our knowledge of the pathophysiology of schizophrenia, we might hypothesize a priori that a drug with a certain pharamcological action might prove effective in treating the disease. We then test it in a series of experiments and gain a posteriori knowledge.
But modern medical research often turns this paradigm on its head -- we are increasingly gathering vast amounts of data without a specific hypothesis and then utilizing it to formulate a hypothesis afterwards. For instance, we might utilize functional neuroimaging to study the hippocampi of a number of people, some with or without schizophrenia, and then based on the results formulate a hypothesis about how various regions of the hippocampus interact in schizophrenia.
But, of course, even this is an oversimplification. In truth -- and again, I'm no expert on epistemology -- a priori and a posteriori knowledge seem to interact back and forth in medical science, and future discoveries will depend on being able to utilize both modes of reasoning in a complimentary fashion.
No, brains are not a way to think about mental life, they are the explanation of mental life. If a therapy works to change behavior, then it's because it affects the brain.
The current state of cognitive neuroscience does not yet allow us to satisfactorily link mental phenomena with clearly delineated activity in the brain. I think your comment above is a bit glib. Obviously, the brain is the substrate for mental activity, but that doesn't mean that anything that doesn't have a brain-based explanation (yet) is specious.
For instance,the complex concept of the ego is central to many psychotherapies that have been demonstrated to have clinical benefits. Yet you cannot take an MRI film and point to the ego for me. We cannot ablate someone's ego -- it's not like Wernicke's area. Does that mean we should dismiss the ego as Freudian nonsense?
An overly mechanistic attitude towards the mind-brain relationship (e.g., "brains are the explanation of mental life") will lead us down an intellectually stunted road akin to 19th century phrenology. An overly abstract attitude towards the mind-brain relationship leads us to Dr. Phil. Freud was a neurologist by training and, utilizing the limited scientific knowledge of his time, continually sought to revise and amend his theories based on clinical data. We should perhaps remember that.
Sorry for the long post, everyone!