infima species (Latin ‘lowest species’), a species that is not a genus of any other species. According to the theory of classification, division, and definition that is part of traditional or Aristotelian logic, every individual is a specimen of some infima species. An infima species is a member of a genus that may in turn be a species of a more inclusive genus, and so on, until one reaches a summum genus, a genus that is not a species of a more inclusive genus. Socrates and Plato are specimens of the infima specis human being (mortal rational animal), which is a species of the genus rational animal, which is a species of the genus animal, and so on, up to the summum genus substance. Whereas two specimens of animal – e.g., an individual human and an individual horse – can differ partly in their essential characteristics, no two specimens of the infima species human being can differ in essence. See also ARISTOTLE , ESSENTIALISM , GENUS GEN- ERALISSIMUM , TREE OF PORPHYR. W.E.M.