An Argumnet Against Pure Materialism

Would you try to apply psychology to the field of physics? (If so, then you're about four centuries behind modern science, and should probably be exterminated for the betterment of the human race.) Why then try to apply physics to the field of psychology?

I understand that certain emotions corrolate to certain physical processes and that our thoughts in general are influenced by our emotions (if I haven't eaten all day I get irritated; when irritated my thoughts turn dark). My question is: how do these two entirely seperate ideas mutate into a single idea of every single thought being the direct result of some charge being shot off in the brain? I can understand the influence played, but not the direct law of causality being involved.