Slides / Bullets

• The material conditional →
  • → is a binary truth-functional connective.
  • It has the following truth-table:
  • The sentence on the left of → is called the antecedent, the one on the right is called the consequent.
• • P→Q is tautologically equivalent to ¬P ∨ Q, as can be easily verified using a truth table.
    • So having → in the language doesn’t let us express anything we couldn’t have expressed without it—but it’s convenient nevertheless.
• Translations
  • Suppose I say: ‘If I left my scarf in the coffee shop, I left my cellphone there too’
    • If I left the scarf there and didn’t leave the cellphone there, it’s clear that I’ve said something false. If I left both of them there, it seems pretty clear that I haven’t.
    • What if it turns out I didn’t leave the scarf there? In this case it sounds a bit odd to suggest that I’ve said something false: I might have had no good reason to say what I said, but that’s not the same thing.
• • So there’s a case to be made that ‘P → Q’ is a correct translation into FOL of an English sentence ‘If P, then Q’.
    • Think of it in terms of what one rules out: in saying ‘If P then Q’, one is ruling out the case where P is true and Q isn’t, and it’s not clear that one is ruling out anything else.
  • When we’re doing translations in this course, we will translate ‘If..then...’ using the material conditional.
• • But is this really correct? If it were, the following sentences would all be true:
    • ‘If pigs can fly, the moon is made of green cheese’
    • ‘If pigs can fly, the moon isn’t made of green cheese’
    • ‘If pigs can fly, pigs can’t fly’
  • This seems pretty strange!
• • On the other hand, there’s some evidence that ‘if...then...’ really does express the material conditional.
    • The argument ‘P or Q; therefore if not-P, then Q’ seems valid.
    • But if this is valid, so is ‘not-P or Q; therefore if P then Q’. So the English conditional is true whenever the material conditional is.
  • A vexed question in ‘philosophical logic’.
• • Other English expressions we’ll translated using ‘P→Q’:
    • Q if P (this is obviously equivalent to ‘If P then Q’
    • Q provided that P
    • P only if Q
      • ‘You will pass the course only if you pass the final exam’
  • ‘Unless P, Q’ and ‘Q unless P’ are translated as ‘¬P→Q’
• • It’s important to distinguish the conditional symbol—which is part of FOL—from the notion of logical consequence which is a relation between sentences of FOL.
  • A conditional can be true even if the consequent is not a logical consequence of the antecedent.
  • However, for a conditional is logically true, the consequent does have to be a logical consequence of the antecedent.
• The material biconditional ↔
  • → is a binary truth-functional connective.
  • It has the following truth-table:
  • The biconditional is true when the left hand side and right hand side have the same truth-value; otherwise it’s false.
• • P↔Q is tautologically equivalent to (P→Q)∧(Q→P).
    • It’s also tautologically equivalent to (P∧Q)∨(¬P∧¬Q).
• • We use ‘↔’ to translate the English expression ‘if and only if’, often abbreviated by mathematicians and philosophers as ‘iff’.
    • ‘Iff’ is sometimes read as ‘just in case’—a special bit of jargon.