Elizabeth Olsen

What is the relationship between logic and thought? One view is that logic merely describes how people think. But this view – called 'psychologism' – cannot be quite right. Logic cannot describe how people reason, because although people can reason well, they can also reason badly. The obvious response is to say that logic does not describe how people do think, but rather prescribes how they ought to think. If logic describes how people ought to reason, then if the premises of a logical argument imply the conclusion of that argument and you believe the premises, then you ought to believe the conclusion. According to classical logic the premise, 'grass is green' implies the conclusion, 'the sky is blue or the sky is not blue', but it seems absurd to say that because I believe that grass is green, I ought to believe that the sky is blue or the sky is not blue. What has gone wrong here? Should the principle, 'if the premises imply the conclusion and you believe the premises, then you ought to believe the conclusion' be changed? If so, perhaps it would be more correct to say, 'if the premises imply the conclusion and you believe the premises, then you have reason to believe the conclusion'. Or is classical logic to blame? Are we mistaken in thinking that 'grass is green' implies 'the sky is blue or the sky is not blue'? I examine a number of arguments that relate to this question. I argue that classical logic deserves a philosophy of logic that does not imply that classical logic is not proper logic.