 According to Theophrastus, Alcmaeon was the first Greek thinker to distinguish between sense perception and understanding and to use this distinction to separate animals, which only have sense perception, from humans, who have both sense perception and understanding (DK, B1a, A5). Alcmaeon is also the first to argue that the brain is the central organ of sensation and thought (DK, A5, A8, A10). There is no explicit evidence, however, as to what Alcmaeon meant by understanding. The word translated as understanding here is suniêmi, which in its earliest uses means “to bring together,” so that it is possible that Alcmaeon simply meant that humans are able to bring the information provided by the senses together in a way that animals cannot (Solmsen 1961, 151). Animals have brains too, however, and thus might appear to be able to carry out the simple correlation of the evidence from the various senses, whereas the human ability to make inferences and judgments (DK, B1) appears to be a more plausible candidate for the distinctive activity of human intelligence. It is possible that we should use a passage in Plato’s Phaedo (96a-b = A11) to explicate further Alcmaeon’s epistemology. The passage is part of Socrates’ report of his early infatuation with natural science and with questions such as whether it is the blood, or air, or fire with which we think. He also reports the view that it is the brain that furnishes the sensations of hearing, sight, and smell. This corresponds very well with Alcmaeon’s view of the brain as the central sensory organ and, although Alcmaeon is not mentioned by name, many scholars think that Plato must be referring to him here. Socrates connects this view of the brain with an empiricist epistemology, which Aristotle will later adopt (Posterior Analytics 100a3 ff.). This epistemology involves three steps: first, the brain provides the sensations of hearing, sight and smell, then, memory and opinion arise from these, and finally, when memory and opinion achieve fixity, knowledge arises. Some scholars suppose that this entire epistemology is Alcmaeon’s (e.g., Barnes 1982, 149 ff.), while others more cautiously note that we only have explicit evidence that Alcmaeon took the first step (e.g., Vlastos 1970, 47, n.8).