All mentals processes derive from operations of the brain
Genes determine neuronal function
Social and developmental factors contribute importantly to the variance in mental illness. These factors express themselves in altered Gene expression. Nurture is ultimatelly expressed as nature.
Altered gene expression induced by learning gives rise to changed patterns of neuronal connections, which give rise to different forms of thinking and behaviour.
Psychotherapy produces changes in long-term behaviour by learning which produces changes in gene expression, and hence changes in neuronal interconnection.