While it was once believed that the sort of psychotherapy appropriate for the treatment of neuroses caused by environmental factors is also appropriate for schizophrenia and other psychoses, it is now known that these latter, more serious forms of mental disturbance are best treated by biochemical - that is, medicinal - means.
This is conclusive evidence that psychoses, unlike neuroses have nothing to do with environmental factors but rather are caused by some sort of purely organic condition, such as abnormal brain chemistry or brain malformations.

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in the book it states that the conclusion is:
This is conclusive evidence that psychoses, unlike neuroses have nothing to do with environmental factors but rather are caused by some sort of purely organic condition, such as abnormal brain chemistry or brain malformations.

however, i think that the conclusion should be : it is now known that these latter, more serious forms of mental disturbance are best treated by biochemical - that is, medicinal - means

By using mgmat's "therefore" conclusion method: BECAUSE of the conclusive evidence about phychoses and the environmental factors, THEREFORE more serious forms of mental disturbance are best treated by medicinal means!

this is from PowerScore Critical Reasoning bible, page 40.

please let me know where i am wrong!

thank you ahead of time sir or miss