Experiment alone crowns the efforts of medicine, experiment limited only by the natural range of the powers of the human mind. Observation discloses in the animal organism numerous phenomena existing side by side, and interconnected now profoundly, now indirectly, or accidentally. Confronted with a multitude of different assumptions the mind must guess the real nature of this connection.
---Ivan Petrovich Pavlov (1849 - 1936). Experimental Psychology and Other Essays, Pt. X