44 ideas
16477 | Asserting not-p is saying p is false [Russell] |
13520 | A 'tautology' must include connectives [Wolf,RS] |
13524 | Deduction Theorem: T∪{P}|-Q, then T|-(P→Q), which justifies Conditional Proof [Wolf,RS] |
13521 | Universal Specification: ∀xP(x) implies P(t). True for all? Then true for an instance [Wolf,RS] |
13522 | Universal Generalization: If we prove P(x) with no special assumptions, we can conclude ∀xP(x) [Wolf,RS] |
16484 | There are four experiences that lead us to talk of 'some' things [Russell] |
13523 | Existential Generalization (or 'proof by example'): if we can say P(t), then we can say something is P [Wolf,RS] |
5745 | Quine says quantified modal logic creates nonsense, bad ontology, and false essentialism [Melia on Quine] |
13529 | Empty Set: ∃x∀y ¬(y∈x). The unique empty set exists [Wolf,RS] |
13526 | Comprehension Axiom: if a collection is clearly specified, it is a set [Wolf,RS] |
16486 | The physical world doesn't need logic, but the mental world does [Russell] |
13534 | In first-order logic syntactic and semantic consequence (|- and |=) nicely coincide [Wolf,RS] |
13535 | First-order logic is weakly complete (valid sentences are provable); we can't prove every sentence or its negation [Wolf,RS] |
8789 | Various strategies try to deal with the ontological commitments of second-order logic [Hale/Wright on Quine] |
2947 | Questions wouldn't lead anywhere without the law of excluded middle [Russell] |
16479 | 'Or' expresses hesitation, in a dog at a crossroads, or birds risking grabbing crumbs [Russell] |
16480 | A disjunction expresses indecision [Russell] |
16483 | Disjunction may also arise in practice if there is imperfect memory. [Russell] |
16481 | 'Or' expresses a mental state, not something about the world [Russell] |
16487 | Maybe the 'or' used to describe mental states is not the 'or' of logic [Russell] |
13519 | Model theory uses sets to show that mathematical deduction fits mathematical truth [Wolf,RS] |
13531 | Model theory reveals the structures of mathematics [Wolf,RS] |
13532 | Model theory 'structures' have a 'universe', some 'relations', some 'functions', and some 'constants' [Wolf,RS] |
13533 | First-order model theory rests on completeness, compactness, and the Löwenheim-Skolem-Tarski theorem [Wolf,RS] |
13537 | An 'isomorphism' is a bijection that preserves all structural components [Wolf,RS] |
13539 | The LST Theorem is a serious limitation of first-order logic [Wolf,RS] |
13538 | If a theory is complete, only a more powerful language can strengthen it [Wolf,RS] |
13525 | Most deductive logic (unlike ordinary reasoning) is 'monotonic' - we don't retract after new givens [Wolf,RS] |
16475 | A 'heterological' predicate can't be predicated of itself; so is 'heterological' heterological? Yes=no! [Russell] |
13530 | An ordinal is an equivalence class of well-orderings, or a transitive set whose members are transitive [Wolf,RS] |
13518 | Modern mathematics has unified all of its objects within set theory [Wolf,RS] |
16966 | Philosophers tend to distinguish broad 'being' from narrower 'existence' - but I reject that [Quine] |
16965 | All we have of general existence is what existential quantifiers express [Quine] |
16963 | Existence is implied by the quantifiers, not by the constants [Quine] |
16964 | Theories are committed to objects of which some of its predicates must be true [Quine] |
4216 | Express a theory in first-order predicate logic; its ontology is the types of bound variable needed for truth [Quine, by Lowe] |
18966 | Ontological commitment of theories only arise if they are classically quantified [Quine] |
14490 | You can be implicitly committed to something without quantifying over it [Thomasson on Quine] |
16961 | In formal terms, a category is the range of some style of variables [Quine] |
16482 | All our knowledge (if verbal) is general, because all sentences contain general words [Russell] |
4758 | Naïve realism leads to physics, but physics then shows that naïve realism is false [Russell] |
16476 | For simple words, a single experience can show that they are true [Russell] |
16485 | Perception can't prove universal generalisations, so abandon them, or abandon empiricism? [Russell] |
16478 | A mother cat is paralysed if equidistant between two needy kittens [Russell] |