Both are correct, somewhat. A great example is brain chemistry. The current ideology in medicine is that brain chemistry creates emotions and thoughts, and if we alter those chemicals we can change those emotions and thoughts. This is virus theory. This alone is stupid. The other (closer to correct) outlook is that the chemicals in our brain are created BECAUSE we feel and think these things. The chemicals are the byproduct, not the emotions and thoughts. So altering those chemicals is useless (this is the part that is stupid). Both are true in part. The chemicals in your brain ARE a byproduct of the mind(terrain theory), but external forced changes to those chemicals ABSOLUTELY changes those thoughts and emotions(viral theory). Otherwise drugs wouldn't work.