Human reasoning operates primarily on the collection of ideas of which the person is immediately conscious. Other ideas are in the background and come into consciousness by various processes. Because reasoning is so often nonmonotonic, conclusions can be reached on the basis of the ideas in consciousness that would not be reached if certain additional ideas were also in consciousness. Human emotions influence human thought by influencing what ideas come into consciousness. For example, anger brings into consciousness ideas about the target of anger and also about ways of attacking this target.
Human emotions are strongly related to blood chemistry.
Hormones and neurotransmitters belong to the same family of substances. The sight of something frightening puts certain substances in our blood streams, and these substances may reduce the thresholds of synapses where the dendrites have receptors for these substances. A design that uses environmental or internal stimuli to bring whole classes of ideas into consciousness is entirely appropriate for a lower animals. We inherit this mechanism from our animal ancestors. According to these notions, paranoia, schizophrenia, depression and other mental illnesses would involve malfunctions of the chemical mechanisms that bring ideas into consciousness.