Abstract10.1 We will be using a particular form of natural deduction, known as the method of subordinate proofs, to construct proofs in the system of logic which is described in chapters II to V. There are two reasons for using the method of subordinate proofs: one is pragmatic and the other is technical. Experience has shown that the method of subordinate proofs is easy to teach and facilitates the construction of proofs. The method of subordinate proofs allows us to construct proofs which have other proofs as hypothesis in a straightforward manner.
10.2 In order to make this book self-contained, this chapter is an introduction to the method of subordinate proofs. So that the reader may concentrate on the method, we will sketch a subordinate proof formulation of a system of logic with which he is already familiar: the propositional calculus. Our discussion is quite informal. The reader is encouraged to provide the missing details using his favorite formulation of the propositional calculus.
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