 Alcmaeon of Croton was an early Greek medical writer and philosopher-scientist. His exact date, his relationship to other early Greek philosopher-scientists, and whether he was primarily a medical writer/physician or a typical Presocratic cosmologist, are all matters of controversy. He is likely to have written his book sometime between 500 and 450 BCE. The surviving fragments and testimonia focus primarily on issues of physiology, psychology, and epistemology and reveal Alcmaeon to be a thinker of considerable originality. He was the first to identify the brain as the seat of understanding and to distinguish understanding from perception. Alcmaeon thought that the sensory organs were connected to the brain by channels (poroi) and may have discovered the poroi connecting the eyes to the brain (i.e. the optic nerve) by excising the eyeball of an animal, although it is doubtful that he used dissection as a standard method. He was the first to develop an argument for the immortality of the soul. He used a political metaphor to define health and disease: The equality (isonomia) of the opposing powers which make up the body (e.g., the wet, the dry, the hot, the cold, the sweet, the bitter etc.) preserves health, whereas the monarchy of any one of them produces disease. Alcmaeon discussed a wide range of topics in physiology including sleep, death, and the development of the embryo. It is unclear whether he also presented a cosmology in terms of opposing powers, but we do have some testimonia concerning his views on astronomy. Alcmaeon had considerable impact on his successors in the Greek philosophical tradition. Aristotle wrote a treatise responding to him, Plato may have been influenced by his argument for the immortality of the soul, and both Plato and Philolaus accepted his view that the brain is the seat of intelligence.