Blackwell Reference Online

Extract

To be rational is to be guided by legitimate reasoning. But different criteria of legitimacy are normally considered appropriate for different types of reasoning and so at least nine types of rationality, or roles for the faculty of reason, seem to be commonly recognized in Western culture. First, there is the rationality that consists in conformity with the laws of deductive logic. Thus it would be termed rational (as an instance of the law termed ‘ modus ponens ’) to infer ‘The streets are wet’ from the two premisses ‘It is raining’ and ‘If it is raining, the streets are wet’, whereas it would be irrational to infer ‘The streets are not wet’ from those premisses. The exact extent of such laws is, however, controversial. For example, deductive logic may or may not be conceived to include laws not only about inferences depending on propositional connectives like ‘if and ‘or’, or on quantifiers like ‘some’ and ‘all’, but also about those depending on modalities such as ‘possibly’ and ‘necessarily’. A second form of rationality is exhibited by correct mathematical calculations. Thus it is rational to infer ‘x > 12’ from the premisses ‘ x is a prime number’ and ‘ x > 11’. Of course, this has nothing to do with the technical sense in which numbers, like the square root of 2, that are not equivalent to the ratio of one integer to another, are termed ‘irrational’ and others ‘rational’. ... log in or subscribe to read full text