Aristotle incorrectly observed the absence of the kidney in fish and birds and deduced that it was not essential for the existence of a living organism. This underlies his observations on structure and function of the kidney. From examination of rhesus monkeys he generalized that the right kidney is higher than the left. Aristotle did not consider that the renal pelvis is divided by a filter membrane into 2 chambers, and wrote that no blood reaches the renal pelvis. The theory of the ‘filter kidney’ cannot thus be attributed to Aristotle. The function of the kidney was described as being to separate the surplus liquid from the blood inside the renal meat (not in the renal pelvis) and to transform this liquid into what Aristotle called residuum, i.e. the urine. Aristotle also considered that the kidneys acted to anchor the blood vessels to the body. He only briefly considered renal pathol