Opponent process is observable in neuro impulses (Garbor wavelets ), color illusions of seeing an opposite color after habituation, physiological homeostatis reactions for temperature, oxygen, food, and stimuluation control. Drug addiction is best understood as being due to the modulation of a homeostatis into a positive and negative phase. Any stimulus, drug, or behavior will lose its strength due to learned habuation but the opponent reaction to the original drug remains strong. In terms of drugs this means that eventual people who started to take heroin to experience pleasure will be taking it to feel neutral.