In order to make this argument, it is necessary to assume something, as little as possible, about human emotions. Here are some points. 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. According to these notions, paranoia, schizophrenia, depression and other mental illnesses would involve malfunctions of the chemical mechanisms that gate ideas into consciousness. A paranoid who believes the CIA is following him and influencing him with radio waves can lose these ideas when he takes his medicine and regain them when he stops. Certainly his blood chemistry cannot encode complicated paranoid theories, but they can bring ideas about threats from wherever or however they are stored. Hormones analogous to neurostransmitters open synaptic gates to admit whole classes of beliefs into consciousness.