Not necessary, because even if not all neurochemical changes produce psychological changes, the changes that occur in connection with “talk” therapy still can produce psychological changes. The author never reasoned, “Because therapy produces chemical changes in the brain, those chemical changes must cause psychological changes.” Rather, the author pointed out that the chemical changes “seem to correspond to improvements in certain aspects of the patient’s behavior.” It’s this association with improvements that the author finds to be supportive of the causal connection between these particular chemical changes and psychological changes. Other chemical changes might not be associated with improvement in behavior, and those other chemical changes then don’t have to produce any psychological changes.

Necessary, because if this weren’t true — if improvements in a patient’s behavior produced by “talk” therapy does NOT occur only through chemical changes — then we have no reason to think that drugs that affect the brain’s neurochemistry can be just as effective as “talk” therapy. “Talk” therapy may be more effective, because it can work through non-chemical means that aren’t available by taking drugs.

Not necessary, because the argument is concerned with whether drugs can be as effective as “talk” therapy. Whether “talk” therapy is effective or not has no bearing on whether drugs can equal “talk” therapy’s effect, however strong or weak it may be.

Not necessary, because it’s too broad. The author tries to conclude that drugs will be as effective as “talk” therapy concerning improving the behavior of patients with psychological disorders. This conclusion does not encompass the more general claim that psychology and neuroscience will become indistinguishable.

Not necessary, because the argument’s reasoning doesn’t require any belief about cost. The conclusion is about effectiveness, not about affordability or accessibility.