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'What about inhibitory neurons? Why did they evolve? Consider the simple task of a coral polyp opening or closing its mouth. For its mouth to open, one set of muscles must contract and another must relax. And the converse for closing its mouth. The existence of both excitatory and inhibitory neurons enabled the first neural circuits to implement a form of logic required for reflexes to work. They can enforce the rule of “do this, not that,” which was perhaps the first glimmer of intellect to emerge from circuits of neurons. “Do this, not that” logic was not new—such logic already existed in the protein cascades of single cells. But this ability was recapitulated in the medium of neurons, which made such logic newly possible on the scale of level-three multicellular life. Inhibitory neurons enabled the necessary inner logic for catch-and-swallow reflexes to work.' (Uit Bennett, M, A Brief History of Intelligence.)