The Rational Animal
Most human behavior arises out of our instinctive or emotional responses. We don't save rational thought for solving algebra problems or crossword puzzles, though. We also use it in our moral and social reasoning, but it's implicit in our neural design that the logical elements of such thinking are literally an afterthought. Initial processing of such decisions comes from the evolutionarily older emotional and instinctive parts of the brain, with the higher level logical thinking parts of the prefrontal cortex activated later as a sort of CYA lawyer to check that all the legal niceties are observed.