There is a developing awareness amongst neuro-psychology that the emotions play an important part in our reasoning skills, and that it is no longer possible, even in principle, to consider rationality as something separate from our body.
Interesting research has been undertaken into the predicament of patients suffering from anasognosia, which is an inability to experience emotion, although the disorder leaves rationality (logic) and linguistic abilities intact. In one case, an investigator was discussing with a patient the possibility of a meeting at a later date, and gave the patient the option of choosing between two dates. The patient then began analysing which of the two dates would be preferable and considered the pros and cons of each in considerable detail. In fact, the consideration only stopped - after half an hour of thought - when the investigator himself stated a preference for one of the dates.
The body, particularly the emotions, play a central part in our reasoning capacity, most importantly when it comes to making decisions. An example may make things clearer: in playing chess there are an extremely large number of possible moves. A normal player will automatically exclude certain moves from consideration, for example those which lead directly to the loss of a queen, thus winnowing down the number of options that have to be considered. In practice, the player will consider only a small handful of potential moves, and the choice amongst those options will depend upon a wide variety of factors, including previous training and experience, the understanding of the opponent's abilities and temperament, and the mood of the player concerned. These decisions are ultimately based upon the emotions, which play this role within normal human reasoning. When the brain is considering certain courses of action it ‘presents’ the outcome to the body, and makes decisions based on how the body reacts. In the example of a chess match, the player concerned will envisage a particular move, and imagine the situation that would result (better players imagine the situation that would result after more moves). In playing the game, assuming a desire to win, certain situations will be desired more than others. For example, a strong pawn structure and well developed pieces will be seen as desirable or valuable, and a situation which results in the loss of a queen will - other things being equal - be seen as very undesirable and lacking in value.
In saying that certain outcomes are desired or not desired, or are seen as more or less valuable, what is at issue is the emotional weight given to those different imagined scenarios. The player will physically react to those scenarios, and a decision will be reached based on that reaction. Our decision making capacity rests upon our biological nature - our existence as homo sapiens, with all the biological heritage consequent upon that fact. We are human beings, not simply rational intellects, and as such we have an embodied existence – our intellects are dependent upon our biology. As Antonio Damasio, one of the principal researchers in this field, puts it: ‘It does not seem sensible to leave emotions and feelings out of any overall concept of mind’ – in other words, it is an illusion to think that we can make decisions without regard to our emotions; on the contrary, full rationality requires a comparably complete emotional engagement. Rationality is dependent upon our emotional development, or, as Hume put it, our reason is a slave to our passions.