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Roy Harris should practise what he preaches ("The decline of reason", 15 May).

By dismissing mathematical logic as "no use to man or beast", he betrays his own misologist tendencies and falls into the elementary error of concluding that because he cannot see any use for a thing, therefore it has no use.

Mathematicians unfortunately confront this error of reasoning daily, in our students, in our family and friends, and even in colleagues who should know better. In fact, of course, mathematical logic has many uses, one of which is to clarify one's thinking to avoid mistakes of this kind. Another is to understand the process of reasoning in sufficient detail in order to tell a computer how to do it - which is one of the reasons why mathematical logic is on the syllabus of every computer science degree worth its salt.

Robert Wilson, Professor of pure mathematics, Queen Mary, University of London.