green numbers give full details | back to texts | unexpand these ideas
14352 | '¬', '&', and 'v' are truth functions: the truth of the compound is fixed by the truth of the components |
Full Idea: It is widely agreed that '¬', '&', and 'v' are 'truth functions': the truth value of a compound sentence formed using them is fully determined by the truth value or values of the component sentences. | |||
From: Frank Jackson (Conditionals [2006], 'Equiv') | |||
A reaction: A candidate for not being a truth function might be a conditional →, where the arrow adds something over and above the propositions it connects. The relationship has an additional truth value? Does A depend on B? |
14360 | Possible worlds for subjunctives (and dispositions), and no-truth for indicatives? |
Full Idea: Subjunctive conditionals are intimately connected with dispositional properties and causation. ...Consequently, a position some find attractive is that possible worlds theory applies to subjunctives, while the no-truth theory applies to indicatives. | |||
From: Frank Jackson (Conditionals [2006], 'Indicative') | |||
A reaction: My intuitions are to reject this and favour a unified account, where both sorts of conditionals are mappings of the relationships among the facts of actuality. Nice slogan! |
14355 | (A&B)→A is a logical truth, even if antecedent false and consequent true, so it is T if A is F and B is T |
Full Idea: (A&B)→A is a logical truth, but A can be true and B false, so that (A&B) is false. So some conditionals with false antecedent and true consequent are true. If → is a truth function, then whenever A is false and B is true (A→B) is true. | |||
From: Frank Jackson (Conditionals [2006], 'Equiv') | |||
A reaction: This is his third and final step in showing the truth table of → if it is truth functional. |
14354 | When A and B have the same truth value, A→B is true, because A→A is a logical truth |
Full Idea: (A→A) is a logical truth, so some conditionals with antecedent and consequent the same truth value are true. But if '→' is a truth function, that will be true for all cases. Hence whenever A and B are alike in truth value, (A→B) is true. | |||
From: Frank Jackson (Conditionals [2006], 'Equiv') | |||
A reaction: His second step in demonstrating the truth table for →, assuming it is truth functional. |
14353 | Modus ponens requires that A→B is F when A is T and B is F |
Full Idea: Modus ponens is intuitively valid, but in A,A→B|B if A is true and B is false that must be because A→B is false. So A→B is false when A is true and B is false. | |||
From: Frank Jackson (Conditionals [2006], 'Equiv') | |||
A reaction: This is his first step in showing how the truth functional account of A→B acquires its truth table. If you are giving up the truth functional view of conditionals, presumably you are not also going to give up modus ponens? |
14358 | In the possible worlds account of conditionals, modus ponens and modus tollens are validated |
Full Idea: In the possible worlds account modus ponens is validated (the closest world, the actual, is a B-world just if B is true), and modus tollens is validated (if B is false, the actual world is not an A-world, so A is false). | |||
From: Frank Jackson (Conditionals [2006], 'Famous') | |||
A reaction: [see Jackson for slightly fuller versions] This looks like a minimal requirement for a decent theory of conditionals, so Jackson explains the attractions of the possible worlds view very persuasively. |
14359 | Only assertions have truth-values, and conditionals are not proper assertions |
Full Idea: In the no-truth theory of conditionals they have justified assertion or acceptability conditions but not truth conditions. ...The motivation is that only assertions have truth values, and conditionals are arguments, not proper assertions. | |||
From: Frank Jackson (Conditionals [2006], 'No-truth') | |||
A reaction: Once I trim this idea down to its basics, it suddenly looks very persuasive. Except that I am inclined to think that conditional truths do state facts about the world - perhaps as facts about how more basic truths are related to each other. |
14357 | Possible worlds account, unlike A⊃B, says nothing about when A is false |
Full Idea: In the possible worlds account of conditionals A⊃B is not sufficient for A→B. If A is false then A⊃B is true, but here nothing is implied about whether the world most like the actual world except that A is true is or is not a B-world. | |||
From: Frank Jackson (Conditionals [2006], 'Possible') | |||
A reaction: The possible worlds account seems to be built on Ramsey's idea of just holding A true and seeing what you get. Being committed to B being automatically true if A is false seems highly counterintuitive. |
14356 | We can't insist that A is relevant to B, as conditionals can express lack of relevance |
Full Idea: One addition to the truth functional account of conditionals is that A be somehow relevant to B. However, sometimes we use conditionals to express lack of relevance, as in 'If Fred works he will fail, and if Fred doesn't work he will fail'. | |||
From: Frank Jackson (Conditionals [2006], 'Possible') | |||
A reaction: This certainly seems to put paid to an attractive instant solution to the problem. |