They undoubtedly exist, as they're based on biochemical pathways. You can't necessarily change that, at least easily. I don't think that's the point hes trying to make, however, as he does have a point in the willpower effect-- it's basically the same as the placebo effect; your belief in something can have a positive chemical effect upon your bodies pathways. In the case of drug users, this can be seen in the "bottoming out" effect-- the addiction pathway doesn't suddenly flip, in fact in almost all cases it always remains permanently on, rather it's the person and their willpower who decides that they've had enough.
The human mind is an amazing thing, especially in its ability to either alter or deal with modified pathways caused by external stress factors.