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Definition:

‘According to this picture, to be rational is to reason in accordance with principles of reasoning that are based on rules of logic, probability theory, and so forth. If the standard picture of reasoning is right, principles of reasoning that are based on such rules are normative principles of reasoning, namely they are the principles we ought to reason in accordance with.’ (Stein 1996, 4)

Example of deductive reasoning:

Jessie believes that either Paula got an A in geometry or Vanessa did, and also that Vanessa did not get an A but got a D instead. From these premises, Jessie concludes that it is Paula who got an A. Jessie is rational: her logical reasoning is in accordance with a rule of logic that one can infer P from (P or Q and not Q) (disjunctive syllogism).

Example of probabilistic reasoning:

Felix assigns the probability 0.8 to the idea that it will rain tomorrow based on the televised weather forecast that there is an 80% chance of rain tomorrow. Felix also assigns the probability 0.9 to the idea that it will rain tomorrow and so he will go to the gym for a workout. Felix is irrational: his probabilistic judgment violates what is known as the ‘conjunction rule’; the probability of an event A occurring, P(A), cannot be less than the probability of A and another event B occurring at the same time, P(A&B).

But how often should human agents reason in accordance with these rules to be considered rational agents? If the answer is ‘Always’, then a rational person wouldn’t be allowed to make any mistakes; this requirement is far too demanding. It is obvious that human agents are not rational according to such a stringent requirement. A realistic view would take into account the fact that rational agents can make reasoning errors, such as the occasional performance error that is attributed to some interfering factor (e.g., lack of concentration) (see Box 1B), but they do not make systematic reasoning errors. Thus, rationality is compatible with occasional reasoning errors, but it is not compatible with systematic reasoning errors.