The "Triune Brain Theory" was invented by psychobiologist Paul McLean in the 1950s. It claims that the human skull holds not one brain, but three: the primitive, limbic and neocortex. According to McLean, these brains are "three interconnected biological computers, each with its own special intelligence, its own subjectivity, its own sense of time and space and its own memory". Thus the brains are not just parts of one giant brain, but three separate parts which operate quite separately from one another.

McLean claims the three brains all developed at separate times, and that they lie in order of oldest to youngest from the bottom up, as in a geological stratum of rocks. The three brains, from the middle (oldest) to the outermost (youngest), as suggested by McLean, are as follows:

  • The reptilian, or primitive (archipallium) brain. This is the repetitive, mechanical, paranoid part of the brain. Operating on instinct, the reptilian brain never learns from past mistakes- it's pretty dumb. It controls the basic bodily functions, eating, the beating of the heart, sleep, etc.
  • The limbic , or mammalian (paleopallium) brain. This is the seat of all emotion and pavlovian (learnt) responses. It revolves around avoiding displeasure, or pain, and recreating pleasure and agreeable outcomes of situations. This relatively 'low' brain is also the part which deems an idea to be 'good' or not, based on value judgements.
  • The rational , or neocortex (neopallium) brain. This is the 'newest' brain, and is described as the "thinking cap" of cognitive thinking. The rational brain is responsible for that which separates humans from other animals, and McLean calls it "the mother of invention and father of abstract thought". This is the brain which is divided into left (verbal, problem solvng, linear) and right (abstract, creative, spatial) parts.
The triune brain model McLean hypothesises isn't too different to those suggested in the past: Freud's id, superego and ego worked on a similar basis. Gurdjieff also proposed a three part thinking system, with a brain for the spirit, one for the soul and another for the body. This, in turn, is derivative of other beliefs which claim that humans are made of the head (the actual brain), the soul (found in the heart), and the body (which was primarily found in the belly).

So McLean's not so wacky after all.

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