77 ideas
9535 | 'Contradictory' propositions always differ in truth-value [Lemmon] |
9514 | If A and B are 'interderivable' from one another we may write A -||- B [Lemmon] |
9513 | We write 'P if and only if Q' as P↔Q; it is also P iff Q, or (P→Q)∧(Q→P) [Lemmon] |
9512 | We write the 'negation' of P (not-P) as ¬ [Lemmon] |
9511 | We write the conditional 'if P (antecedent) then Q (consequent)' as P→Q [Lemmon] |
9508 | The sign |- may be read as 'therefore' [Lemmon] |
9509 | That proposition that both P and Q is their 'conjunction', written P∧Q [Lemmon] |
9510 | That proposition that either P or Q is their 'disjunction', written P∨Q [Lemmon] |
9533 | A 'implies' B if B is true whenever A is true (so that A→B is tautologous) [Lemmon] |
9534 | Two propositions are 'equivalent' if they mirror one another's truth-value [Lemmon] |
9516 | A 'well-formed formula' follows the rules for variables, ¬, →, ∧, ∨, and ↔ [Lemmon] |
9517 | The 'scope' of a connective is the connective, the linked formulae, and the brackets [Lemmon] |
9518 | A 'theorem' is the conclusion of a provable sequent with zero assumptions [Lemmon] |
9529 | A wff is 'inconsistent' if all assignments to variables result in the value F [Lemmon] |
9532 | 'Subcontrary' propositions are never both false, so that A∨B is a tautology [Lemmon] |
9528 | A wff is a 'tautology' if all assignments to variables result in the value T [Lemmon] |
9519 | A 'substitution-instance' is a wff formed by consistent replacing variables with wffs [Lemmon] |
9530 | A wff is 'contingent' if produces at least one T and at least one F [Lemmon] |
9531 | 'Contrary' propositions are never both true, so that ¬(A∧B) is a tautology [Lemmon] |
9393 | A: we may assume any proposition at any stage [Lemmon] |
9396 | DN: Given A, we may derive ¬¬A [Lemmon] |
9399 | ∧E: Given A∧B, we may derive either A or B separately [Lemmon] |
9401 | ∨E: Derive C from A∨B, if C can be derived both from A and from B [Lemmon] |
9394 | MPP: Given A and A→B, we may derive B [Lemmon] |
9400 | ∨I: Given either A or B separately, we may derive A∨B [Lemmon] |
9395 | MTT: Given ¬B and A→B, we derive ¬A [Lemmon] |
9402 | RAA: If assuming A will prove B∧¬B, then derive ¬A [Lemmon] |
9397 | CP: Given a proof of B from A as assumption, we may derive A→B [Lemmon] |
9398 | ∧I: Given A and B, we may derive A∧B [Lemmon] |
9525 | We can change conditionals into negated conjunctions with P→Q -||- ¬(P ∧ ¬Q) [Lemmon] |
9524 | We can change conditionals into disjunctions with P→Q -||- ¬P ∨ Q [Lemmon] |
9527 | The Distributive Laws can rearrange a pair of conjunctions or disjunctions [Lemmon] |
9526 | We can change conjunctions into negated conditionals with P→Q -||- ¬(P → ¬Q) [Lemmon] |
9522 | 'Modus ponendo tollens' (MPT) says P, ¬(P ∧ Q) |- ¬Q [Lemmon] |
9523 | De Morgan's Laws make negated conjunctions/disjunctions into non-negated disjunctions/conjunctions [Lemmon] |
9521 | 'Modus tollendo ponens' (MTP) says ¬P, P ∨ Q |- Q [Lemmon] |
9537 | Truth-tables are good for showing invalidity [Lemmon] |
9538 | A truth-table test is entirely mechanical, but this won't work for more complex logic [Lemmon] |
9536 | If any of the nine rules of propositional logic are applied to tautologies, the result is a tautology [Lemmon] |
9539 | Propositional logic is complete, since all of its tautologous sequents are derivable [Lemmon] |
13909 | Write '(∀x)(...)' to mean 'take any x: then...', and '(∃x)(...)' to mean 'there is an x such that....' [Lemmon] |
13902 | 'Gm' says m has property G, and 'Pmn' says m has relation P to n [Lemmon] |
13911 | The 'symbols' are bracket, connective, term, variable, predicate letter, reverse-E [Lemmon] |
13910 | Our notation uses 'predicate-letters' (for 'properties'), 'variables', 'proper names', 'connectives' and 'quantifiers' [Lemmon] |
13904 | Universal Elimination (UE) lets us infer that an object has F, from all things having F [Lemmon] |
13901 | Predicate logic uses propositional connectives and variables, plus new introduction and elimination rules [Lemmon] |
13903 | Universal elimination if you start with the universal, introduction if you want to end with it [Lemmon] |
13906 | With finite named objects, we can generalise with &-Intro, but otherwise we need ∀-Intro [Lemmon] |
13908 | UE all-to-one; UI one-to-all; EI arbitrary-to-one; EE proof-to-one [Lemmon] |
13905 | If there is a finite domain and all objects have names, complex conjunctions can replace universal quantifiers [Lemmon] |
13900 | 'Some Frenchmen are generous' is rendered by (∃x)(Fx→Gx), and not with the conditional → [Lemmon] |
9520 | The paradoxes of material implication are P |- Q → P, and ¬P |- P → Q [Lemmon] |
20547 | We should respect the right of people to live in their own way, even if it is irrational [Swift] |
20564 | Anti-colonial movements usually invoke the right of their 'people' to self-determination [Swift] |
20535 | Isn't it more rational to maximise the average position, but with a safety net? [Swift] |
20537 | Hypothetical contracts have no binding force [Swift] |
20542 | Cosmopolitans reject the right of different states to distribute resources in different ways [Swift] |
20559 | Democracy is bad, but the other systems are worse [Swift] |
20561 | Since all opinions are treated as equal in democracy, it implies there are no right answers [Swift] |
20563 | Design your democracy to yield political stability, or good decisions? [Swift] |
20562 | Design your democracy to treat citizens equally, or to produce better citizens? [Swift] |
20560 | Teledemocracy omits debate and deliberation, which are important parts of good decisions [Swift] |
20554 | Multiculturalism is a barrier to the whole state being a community [Swift] |
20553 | Liberals mistakenly think individuals choose their values, without reference to the community [Swift] |
20556 | The best way to build a cohesive community is to be involved in a war [Swift] |
20557 | Membership and inclusion in a community implies non-membership and exclusion [Swift] |
20555 | Liberals are concerned to protect individuals from too much community [Swift] |
20540 | Redistributing wealth treats some people as means, rather than as ends [Swift] |
20551 | Men have had the power to structure all of our social institutions [Swift] |
20545 | Maybe a freedom is from a restraint, and also in order to do something [Swift] |
20550 | Opportunity should ignore extraneous factors, or foster competence, or ignore all disadvantages [Swift] |
20536 | Inequalities are needed, as incentives to do the most important jobs [Swift] |
20548 | A person can desire redistibution of wealth, without it being for reasons of equality [Swift] |
20541 | You can't necessarily sell your legitimate right to something, even if you produced it [Swift] |
20546 | Libertarians about property ignore the fact that private property is a denial of freedoms [Swift] |
20533 | Justice can be seen as fairness or entitlement or desert [Swift] |
7810 | The 'Eumenides' of Aeschylus shows blood feuds replaced by law [Aeschylus, by Grayling] |