“The problem of universals,” as addressed by Boëthius (480–c.525), has to do with what makes a kind of thing distinct from other things. Take, for example, the domestic dog. Dogs have the greatest genetic variety of any living species. Scientists can now identify every one of them as dogs, from Chihuahuas to Great Danes (in principle, that is—they don’t actually do this), by their DNA, which has certain pre-determined resemblances to earlier lines of canines. However, well before the discovery of genes and DNA, human beings could both identify any particular animal that was a dog as a dog, even though that dog had a unique appearance and personality.

What is true of dogs in this sense is true of all natural species—all of their members seem to share “something.” Plato would have said that a dog’s essence is a copy of an ideal form of dog, in which all dogs “participate.” Aristotle would have said that there is an essence of “dogness,” which can be known to human beings and which is shared by all dogs, but that the dog essence is in each dog and only abstracted by the mind.

Strictly speaking, for Aristotle there does not exist a dog essence apart from Rover, Jake, Lacey, Mirabelle, or any other name that designates a unique animal. The problem of universals is the question of whether Plato or Aristotle was correct. Philosophers have agonized over this question and burnt many candles, oil lamps, and computers in the process. Those who think that the essences in individual things are real have been called realists. Those who think that essences are abstractions or creations of the human mind have been called nominalists.