NEW FOUNDATIONS FOR IMPERATIVE LOGIC III: A General Definition of Argument Validity Peter B. M. Vranas University of Wisconsin-Madison Talk at the University of Warsaw, 17 May 2012

A TAXONOMY OF ARGUMENTS Pure declarative arguments You sinned shamelessly. So: You sinned. Pure imperative arguments Repent quickly. So: Repent. Mixed-premise declarative If you sinned, repent. You will not repent. So: You did not sin. Mixed-premise imperative If you sinned, repent. You sinned. So: Repent. Cross-species declarative Repent. So: You can repent. Cross-species imperative You must repent. So: Repent.

RELATED RESEARCH l New foundations for imperative logic I: Logical connectives, consistency, and quantifiers. Noûs (2008) 42: l In defense of imperative inference. Journal of Philosophical Logic (2010) 39: l New foundations for imperative logic II: Pure imperative inference. Mind (2011) 120: l New foundations for imperative logic IV: Soundness and completeness. In preparation.

OVERVIEW Part 1: THE GENERAL DEFINITION Part 2: CROSS-SPECIES ARGUMENTS Part 3: MIXED-PREMISE ARGUMENTS

PRELIMINARIES l A prescription is an ordered pair of incompatible propositions: the satisfaction proposition and the violation proposition of the prescription. Their disjunction is the context, and its negation is the avoidance proposition of the prescription. l A prescription is unconditional if its context is necessary (e.g., “Repent”) and is conditional otherwise (e.g., “If you sinned, repent”). l One need only consider single-premise pure and cross-species arguments, and two-premise mixed-premise arguments.

MERITING ENDORSEMENT l A typical reason for adducing a valid argument is to convince people that its conclusion is true (if it is a proposition) or that it is supported by reasons (if it is a prescription). l A proposition merits endorsement iff it is true, and a prescription merits endorsement iff it is supported by reasons (facts). l An argument is valid only if, necessarily, if its premises merit endorsement, then its conclusion also merits endorsement.

GUARANTEEING/SUSTAINING l A fact guarantees a proposition P iff, necessarily, if the fact exists, then P is true. l A proposition merits endorsement (i.e., is true) iff it is guaranteed by some fact (e.g., the fact that the proposition is true). Similarly, a prescription merits endorsement iff it is supported by some fact. l Uniform terminology: a fact sustains a proposition P iff it guarantees P, and a fact sustains a prescription I iff it supports I.

PRO TANTO/ALL-THINGS- CONSIDERED ENDORSEMENT l Meriting pro tanto endorsement: being sustained by some fact. Too weak. l Meriting all-things-considered endorsement: being undefeatedly sustained by some fact. l (DP) An argument is valid only if, necessarily, if its premises are sustained by some fact, then its conclusion is also sustained by some fact. l (DA) An argument is valid only if, necessarily, if its premises are undefeatedly sustained by some fact, then its conclusion is also undefeatedly sustained by some fact.

THE GENERAL DEFINITION l General Definition of Argument Validity: An argument is valid iff, necessarily, every fact that sustains every premise of the argument also sustains the conclusion of the argument. l In support of the General Definition (GD): GD (1) entails both DP and DA, and (2) yields as special cases:  Definition 1: P entails P iff, necessarily, every fact that guarantees P also guarantees P.  Definition 2: I entails I iff, necessarily, every fact that supports I also supports I.

PART 2 Part 1: THE GENERAL DEFINITION Part 2: CROSS-SPECIES ARGUMENTS Part 3: MIXED-PREMISE ARGUMENTS

CROSS-SPECIES IMPERATIVE ARGUMENTS l Definition 3 (follows from GD): P entails I iff, necessarily, every fact that guarantees P supports I. l Equivalence Theorem 1: P entails I iff P entails that some fact whose existence follows from P undefeatedly supports I. l E.g., the following argument is valid: The fact that you have sworn to tell the truth is an undefeated reason for you to tell the truth. So: Tell the truth. l Cf. Hume’s thesis and Poincaré’s Principle.

FURTHER CONSEQUENCES OF EQUIVALENCE THEOREM 1 l P entails I only if P entails that some fact undefeatedly supports I. l So the following arguments are not valid: (1)You will tell the truth. So: Tell the truth. (2)There is a reason for you to tell the truth. So: Tell the truth. (3)Jupiter is the largest planet. So: Either go to Jupiter or don’t go to the largest planet.

CROSS-SPECIES DECLARATIVE ARGUMENTS l Definition 4 (follows from GD): I entails P iff, necessarily, every fact that supports I guarantees P. l Equivalence Theorem 2: I entails P iff P follows from the proposition that there is a fact which possibly supports I. l E.g., the following are equivalent (and valid): (1)Marry me. So: Possibly, there is a reason for you to marry me. (2)There is a fact which is possibly a reason for you to marry me. So: Possibly, there is a reason for you to marry me.

FURTHER CONSEQUENCES OF EQUIVALENCE THEOREM 2 The following arguments are not valid: (1)Marry me. So: There is a reason for you to marry me. (2)If he comes, leave the files open. Do not leave the files open. So: He will not come. (3)Let it be the case that: he does not come, and you do not leave the files open. So: He will not come. (4)Marry Dan’s only daughter. So: Dan has only one daughter.

PART 3 Part 1: THE GENERAL DEFINITION Part 2: CROSS-SPECIES ARGUMENTS Part 3: MIXED-PREMISE ARGUMENTS

MIXED-PREMISE DECLARATIVE ARGUMENTS l Definition 6 (follows from GD): {I, P} entails P iff, necessarily, every fact that both supports I and guarantees P guarantees P. l Equivalence Theorem 3: {I, P} entails P iff P follows from the proposition that some fact which guarantees P possibly supports I. l E.g., the following argument is valid: Either repent or undo the past. It is impossible for you to undo the past. So: It is possible for you to repent.

INCONSISTENCY BETWEEN PROPOSITIONS & PRESCRIPTIONS l Definition 7: P and I are inconsistent iff, neces- sarily, no fact both guarantees P and supports I. l Special cases: (1) P is impossible. (2) I is neces- sarily violated. (3) I entails the negation of P. (4) P entails the negation of I. (5) P entails that no fact supports I. l Examples: (3) Repent. It is impossible for you to repent. (4) Repent. The fact that you have sworn not to repent is a conclusive reason for you not to repent. (5) Repent. There is no reason for you to repent.

MIXED-PREMISE IMPERATIVE ARGUMENTS l Definition 8 (follows from GD): {I, P} entails I iff, necessarily, every fact that both supports I and guarantees P supports I. l E.g., the following argument is valid: Disarm the bomb. Every reason to disarm the bomb is a reason to cut the wire. So: Cut the wire. l Theorem 6: If I is unconditional and P is consistent with the proposition that some fact undefeatedly supports I&~I, then {I, P} does not entail I.

INVALID MIXED-PREMISE IMPERATIVE ARGUMENTS  1 (1)Either marry him or dump him. You are not going to marry him. So: Dump him. (2) Either marry him or dump him. You are not going to marry him, and you are not going to dump him. (2a) So: Dump him. (2b) So: Marry him. If (1) is valid, then (2a) and (2b) are also valid. But (2a) and (2b) are not valid: from “Do X or Y” and “You are not going to do X or Y” neither “Do X” nor “Do Y” follows. So (1) is not valid.

INVALID MIXED-PREMISE IMPERATIVE ARGUMENTS  2 (1)If the light is red, stop. The light is red. So: Stop. (2)Stop. So: If the light is green, stop. (3)If the light is red, stop. The light is red. So: If the light is green, stop. If (1) is valid, then (because (2) is valid) (3) is valid. But (3) is not valid. So (1) is not valid.

INVALID MIXED-PREMISE IMPERATIVE ARGUMENTS  3 (1)If you drink, don’t drive. You are going to drink. So: Don’t drive. (2)If you drink, don’t drive. You are going to drink, but the fact that your daughter will die if you don’t drive her to the hospital is an undefeated reason for you to drive and not drink. So: Don’t drive. If (1) is valid, then (2) is also valid. But (2) is not valid. So (1) is not valid.

CONCLUSION: VIRTUES OF THE GENERAL DEFINITION l The General Definition is: (1) general (it provides unification); (2) usable (it yields concrete results); (3) useful (it transmits meriting endorsement); (4) formally acceptable (reflexivity/transitivity); (5) intuitively acceptable; (6) principled (it goes beyond intuition); and (7) in harmony with previous work. l Next step: Introduce a formal language & a proof procedure, and prove soundness & completeness.