Societies would _not_ be better off if everyone were like Mr Spock, all rationality and no emotion. Instead, a balance - a teaming up of the internal rivals - is optimal for brains. ... Some balance of the emotional and rational systems is needed, and that balance may already be optimized by natural selection in human brains.
David Eagleman, Incognito: The Secret Lives of the Brain
balance, emotion, rationality, science