Damasio, Antonio R. 1994. Descartes Error. New York: G. P. Putnams Sons.
"I think, therefore I am....
From this I knew that I was a substance, the whole essence or nature of which is to think, and that for its existence there is no need of any place, nor does it depend on any material thing...
"[L]ong before the dawn of humanity, beings were beings.
There is a common-sense notion of what it is to think about something, and what the qualities are that make up an inner life. The vocabulary used to express this notion deals with imagining or remembering, and feeling emotions about those thoughts. Damasios book reinterprets the folk theory of thinking in a scientific framework, arguing that emotions are both critical to rational behavior and thoroughly grounded in the body.
In the scientific literature, emotions are usually divided into two broad categories: primary and secondary emotions. The highest-level neural mediator of primary emotion is the limbic system, and in particular the amygdala and cingulate gyrus. Emotional responses are usually initiated in these structures in response to percepts, and move from there to the rest of the body, evoking responses in the autonomic nervous system and the production of hormones and neurotransmitters. The evolutionary advantage of this system of emotional coordination is that recognition of a threat or opportunity in the brain results in appropriate preparatory actions in the rest of the body. The release of neurotransmitters also affects brain functions, limiting or facilitating particular types of cognition as the situation demands.
Secondary emotions are controlled primarily by the frontal cortices, which invoke emotional responses in response to thoughts rather than percepts. Instead of regulating its own independent emotional production centers, the frontal cortices send signals to the limbic system to generate an emotional response. These signals may be full-fledged emotions, or may only produce part of the response spectrum, such as neurotransmitter release.
Having a physical emotion in response to a secondary emotional stimuli does have a purely survival value, as when a threat is anticipated before it can be perceived. The impairment of secondary emotional responses can also have a dramatic effect on more abstract adaptations such as social fitness. Damasio explains this with his "somatic marker hypothesis," which posits that when a thought generates a frontal cortex emotional response, the thought is "marked" by the physical emotion. Likewise, the cognitive processing of the thought is affected by the neurotransmitters evoked by the emotion. Further processing of the thought will be inhibited by negative emotions, and facilitated by positive emotions.
People who have suffered extensive brain damage to the regions of the frontal cortices essential for producing secondary emotions may superficially appear to function normally, since they produce all the expected primary emotions to direct percepts. But their lack of secondary emotions causes them to act awkwardly in social situations, and they often make apparently irrational decisions. When tested for reasoning abilities they appear to be normal or even above-normal. Damasio claims the difference between tests of reason and the rationality involved in social interactions or everyday decision-making is that both of the latter are highly ambiguous. According to the somatic marker hypothesis, large branches of thought are pruned away by the negative emotions they evoke, and thoughts that result in social rewards are encouraged by positive emotions. Without emotional markers, brain-damaged patients while away long hours in "irrational" pursuits that would provoke strong feelings of guilt or anxiety in normal people.
Although philosophers have prided the human capacity for rationality, and often conceive of the mind as operating on a plane removed from the body, Damasio argues that the body and its "irrational" emotions are in fact critical to the effective pursuit of reason.