Who knows what this really means, but I believe it is correct.The Greatest Psychologist Of All Time will be the Neurochemist who proves that "psychology" is just chemicals floating around your brain cells.
From (at least) the time of Newton we have understood the nature of cause and effect- and the brain is subject to the same laws. It is a biologic, organ subject to chemical and electrical processes that follow physical laws- evolved to aid in the organisms survival.
Even the early psychologists (like Fr ) understood that solving the puzzle of thinking (brain activity) would eventually come down to understanding its workings, not in theoretical terms, but through absolute biological research.
Having said that, my favorite psychologist began with one basic theory- if you do something once, you are more likely to do it again- unfortunately, I don't remember who that was. Maybe Watson.