39 ideas
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] |
13523 | Existential Generalization (or 'proof by example'): if we can say P(t), then we can say something is P [Wolf,RS] |
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] |
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] |
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] |
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] |
23154 | We divide mankind into friend and foe, and cooperate with one and compete with the other [Russell] |
23153 | Gradually loyalty to a creed increased, which could even outweigh nationality [Russell] |
23152 | Increasingly war expands communities, and unifies them through fear [Russell] |
23155 | In early societies the leaders needed cohesion, but the rest just had to obey [Russell] |
23156 | The economic and political advantages of great size seem to have no upper limit [Russell] |
23157 | Government has a negative purpose, to prevent trouble, and a positive aim of realising our desires [Russell] |
23151 | A monarch is known to everyone in the group, and can thus unite large groups [Russell] |
23167 | Power should be with smaller bodies, as long as it doesn't restrict central powers [Russell] |
23168 | A state is essential, to control greedy or predatory impulses [Russell] |
23163 | In an anarchy universities, research, books, and even seaside holidays, would be impossible [Russell] |
23166 | In democracy we are more aware of being governed than of our tiny share in government [Russell] |
23162 | Managers are just as remote from workers under nationalisation as under capitalism [Russell] |
23165 | Socialists say economic justice needs some state control of industries, and of foreign trade [Russell] |
23160 | Being a slave of society is hardly better than being a slave of a despot [Russell] |
23158 | Slavery began the divorce between the work and the purposes of the worker [Russell] |
23161 | Slaves can be just as equal as free people [Russell] |
23159 | Scarce goods may be denied entirely, to avoid their unequal distribution [Russell] |
23164 | Modern justice is seen as equality, apart from modest extra rewards for exceptional desert [Russell] |
13304 | Learned men gain more in one day than others do in a lifetime [Posidonius] |
20820 | Time is an interval of motion, or the measure of speed [Posidonius, by Stobaeus] |