Bulletin of the Section of Logic Volume 6/4 (1977), pp. 164–168 reedition 2011 [original edition, pp. 164–170] Frederic A. Johnson A NATURAL DEDUCTION RELEVANCE LOGIC The relevance logic (NDR) presented in this paper is the result of an attempt to find a natural deduction development, in the style of I. M. Copi (Introduction to Logic, 4th ed., MacMillan, 1972), for the relevance logic I presented in "A Three-Valued Interpretation for a Relevance Logic" (The Relevance Logic Newsletter, Vol. 1, no. 3, 1976). The propositional variables of NDR are, p1, p2, . . .. NRD's well-formed formulas are constructed in the standard way by using propositional variables, parentheses and the connectives, −, * and ∨, in order of increasing binding strength. 'P ⊃ Q' is by definition '−(P * −Q)'. Capital letters with or without subscripts are metalinguistic variables which range over the well-formed formulas. We will use '`r' to present NDR's rules of inference: 1. P `r P ∨Q, where every pi (Restricted Addition, RA) in Q occurs in P . 2. P `r P * (Q ∨ −Q), where every (Restricted Tautology pi in Q occurs in P . Conjunction, RTC) 3. P,Q `r P *Q (Conjunction, Conj.) 4. P *Q `r P (Simplification, Simp.) 5. P ∨Q *R `r P ∨Q (Disjunctive Simplification, DS) 6. P ∨Q * −Q `r P (Contradiction Elimination, CE) 7. If S ≡l T in virtue of exactly one of the following statements then F (S) ` F (T ). i) P * (Q ∨R) ≡l P *Q ∨ P *R (DeMorgan's, DeM) ii) P * (Q ∨R) ≡l P *Q ∨ P *R (Distribution, Dist.) P ∨Q *R ≡l (P ∨Q) * (P ∨R) A Natural Deduction Relevance Logic 165 iii) P * (Q *R) ≡l (P *Q) *R (Association, Assoc.) P ∨ (Q ∨R) ≡l (P ∨Q) ∨R iv) P *Q ≡l Q * P (Computation, Com.) P ∨Q ≡l Q ∨ P v) −− P ≡l P (Double Negation, DN) vi) P * P ≡l P (Tautology, Taut.) P ∨ P ≡l P NDR's entailment relation, symbolized by '`', is defined as follows: P1, . . . , Pn ` C if and only if there is a sequence of well-formed formulas S1, . . . , Sm such that Sm = C and each Si (1 6 i 6 m) is either a Pi (1 6 i 6 n) or follows from preceding Sj by one of the rules of inference. Theorem 1. If P1, . . . , Pn ` C then P1, . . . , Pn classically entails C and every pi in C occurs in P1, . . . , Pn. Proof. Every valuation which assigns t to the premises of the rules of inference assigns t to the conclusion. Furthermore, none of the rules of inference introduce into the conclusion propositional variables which do not occur in the premises. Theorem 2. (Indirect Proof.) If P *−Q ` R *−R and every pi in Q occurs in P then P ` Q. Proof. Let S1, . . . , Sn be a sequence of well-formed formulae such that S1 = P * −Q, Sn = R * −R and each Si (1 6 i 6 n) is either P * −Q or follows from Sj or from Sj and Sk (1 6 j, k < n). Then construct this sequence of statements: 1. P 2. P * (Q ∨ −Q) 1, RTC a1(= 3). P *Q ∨ P * −Q (P * S ∨ S1) 2, Dist. * * * a2. P *Q ∨ S2 * * * an. P *Q ∨ Sn 166 Frederic A. Johnson an + 1. P *Q an, CE an + 2. Q * P an + 1, Com. an + 3. Q an + 2, Simp. The steps from, but excluding, P *Q ∨ Sj−1 to, and including, P *Q ∨ Sj for 1 < j 6 n are to be filled in as follows: i) If Sj = P * −Q then supply the sequence aj − 1. (P *Q ∨ P * −Q) * (Q ∨ −Q) a1, RTC aj . P *Q ∨ P * −Q aj − 1, Simp. Make aj − 2 = aj−1. ii) If Si ` Sj (i < j) by RA, where Sj = Si∨T , then supply the sequence aj − 1. (P *Q ∨ Si) ∨ T ai, RA aj . P *Q ∨ (Si ∨ T ) aj − 1, Assoc. Make aj − 2 = aj−1. iii) If Si ` Sj (i < j) by RTC, where Sj = Si * (T ∨−T ), then supply the sequence aj − 7. (P *Q ∨ Si) * (T ∨ −T ) ai, RTC aj − 6. (T ∨ −T ) * (P *Q ∨ Si) aj − 7, Com. aj − 5. (T ∨ −T ) * (P *Q)∨ (T ∨ −T ) * Si aj − 6, Dist. aj − 4. (T ∨ −T ) * Si ∨ (T ∨ −T )* (P *Q) aj − 5, Com. aj − 3. (T ∨ −T ) * Si ∨ (P *Q)* (T ∨ −T ) aj − 4,Com. aj − 2. (T ∨ −T ) * Si ∨ (P *Q) aj − 3,DS aj − 1. (P *Q) ∨ (T ∨ −T ) * Si aj − 2, Com. aj . (P *Q) ∨ Si * (T ∨ −T ) aj − 1, Com. Make aj − 8 = aj−1. iv) If Sh, Si ` Sj (h, i < j) by Conj., where Sj = Sh *Si, then supply the sequence aj − 1. (P *Q ∨ Sn) * (P *Q ∨ Si) ah, ai Conj. aj . P *Q ∨ (Sh * Si) aj − 1, Dist. Make aj − 2 = aj−1. Procedures for filling in the lines between aj and aj−1 when Si ` Sj in virtue of Rules 4-7 are also easily constructed. A Natural Deduction Relevance Logic 167 Theorem 3. (Transitivity of Entailment.) If P ` Q and Q ` R then P ` R. Proof. Let S1 (= P ), S2, . . . , Sm (= Q) be a sequence of well-formed formulas which shows that P ` Q and let Sm (= Q), Sm+1, . . . , Sn (= R) be a sequence of well-formed formulas which shows that P ` R. Then S1, . . . , Sn shows that P ` R. Theorem 4. If P classically entails Q and every pi in Q occurs in P then P ` Q. Proof. Assume the antecedent. Then P * −Q is a contradiction. By DeM, Dist., Assoc., Com., DN and Taut. P *−Q ` R1 *−R1 *S1∨ . . .∨Rn * −Rn * Sn * (R1 * −R1 * S1 ∨ . . . ∨Rn * −Rn * Sn is one of the formulas which will be produced when following some of the various mechanical procedures for generating the disjunctive normal form of P * −Q). By CE and Simp. R1 * −R1 * S1 ∨ . . . ∨ Rn * −Rn * Sn ` R1 * −R1. By Theorem 3 (Th. 3), P * −Q ` R1 * −R1. By Th. 2 P ` Q. Theorem 5. (Adjunction). If P ` Q and P ` R then P ` Q *R. Proof. Let S1, . . . , Sm (= Q), . . . , Sn (= R), where m 6 n, be a sequence that shows that P ` Q and P ` R. Let Sn+1 = Q * R. Then S1, . . . , Sn+1 shows that P ` Q *R, using Conj. Theorem 6. (Deduction Theorem). If P *Q and every pi in Q occurs in P then P ` Q ⊃ C. Proof. Assume the antecedent. By Theorem 1 P * Q classically entails C. Then P classically entails Q ⊃ C. Since every pi in Q occurs in P and every pi in C occurs in P *Q it follows that every pi in Q ⊃ C occurs in P . By Theorem 4 P ` Q ⊃ C.1 Theorem 7. (Antilogism). If P *Q ` R and every pi in Q occurs in P then P * −R ` −Q. Proof. By Simp. P *−R ` P . Assume the antecedent. By Th. 6 and the definition of '⊃' P ` −(Q * −R). By Th. 3 P * −R ` −(Q * −R). By Com. 1This proof, suggested by Richard Routley, is more straightforward than my original proof. I am grateful for Professor Routley's comments, which led to several improvements. 168 Frederic A. Johnson and Simp. P * −R ` −R. By Th. 5 P * −R ` −R * −(Q * −R). By Dem, Dist., Com. and Simp. −R * −(Q * −R) ` −Q. By Th. 3 P * −R ` −Q. The difference between NDR and the relevance logic presented in "A Three-Valued Interpretation of a Relevance Logic" is that the latter does not recognize the validity of any arguments with contradictory premises, whereas NDR does. For example, p1 * −p1 ` p1 in NDR. But both of these logics endorse what W. T. Parry (The Logic of C. I. Lewis', The Philosophy of C. I. Lewis, ed. P. A. Schilpp, 1968, pp. 115–54) called the Proscriptive Principle, which keeps those arguments which contain a pi that occurs in the conclusion but not in a premise from being valid. Charles Kielkopf ('Adjunction and Paradoxical Derivations', Analysis, Vol. 35, no. 4, 1975, pp. 127–9) showed that the system which Parry based on the Proscriptive Principle inadvertently permits the derivation of any statement from a contradiction. Perhaps the most worrisome feature of NDR is that it denies that in general if A entails B then −B entails −A. For example, though p1 * p2 entails p1 it is false that −p1 entails −(p1 * p2). But the reservations which beginning students of logic have about the validity of Unrestricted Addition, which would guarantee that −p1 entails −p1 ∨−p2 suggest that this apparent defect may be a virtue.2 Department of Philosophy Colorado State University Fort Collins, Colorado 80523 2I am grateful to Professor Charles Kielkopf and Professor Patrick McKee for their helpful comments.