129 ideas
| 14857 | The highest wisdom has the guise of simplicity [Nietzsche] |
| 14854 | Deep thinkers know that they are always wrong [Nietzsche] |
| 14833 | Comedy is a transition from fear to exuberance [Nietzsche] |
| 14853 | Truth finds fewest champions not when it is dangerous, but when it is boring [Nietzsche] |
| 24075 | Convictions, more than lies, are the great enemy of truth [Nietzsche] |
| 13439 | Venn Diagrams map three predicates into eight compartments, then look for the conclusion [Bostock] |
| 13421 | 'Disjunctive Normal Form' is ensuring that no conjunction has a disjunction within its scope [Bostock] |
| 13422 | 'Conjunctive Normal Form' is ensuring that no disjunction has a conjunction within its scope [Bostock] |
| 13355 | 'Disjunction' says that Γ,φ∨ψ|= iff Γ,φ|= and Γ,ψ|= [Bostock] |
| 13350 | 'Assumptions' says that a formula entails itself (φ|=φ) [Bostock] |
| 13351 | 'Thinning' allows that if premisses entail a conclusion, then adding further premisses makes no difference [Bostock] |
| 13356 | The 'conditional' is that Γ|=φ→ψ iff Γ,φ|=ψ [Bostock] |
| 13352 | 'Cutting' allows that if x is proved, and adding y then proves z, you can go straight to z [Bostock] |
| 13353 | 'Negation' says that Γ,¬φ|= iff Γ|=φ [Bostock] |
| 13354 | 'Conjunction' says that Γ|=φ∧ψ iff Γ|=φ and Γ|=ψ [Bostock] |
| 13610 | A logic with ¬ and → needs three axiom-schemas and one rule as foundation [Bostock] |
| 13846 | A 'free' logic can have empty names, and a 'universally free' logic can have empty domains [Bostock] |
| 13346 | Truth is the basic notion in classical logic [Bostock] |
| 13545 | Elementary logic cannot distinguish clearly between the finite and the infinite [Bostock] |
| 13822 | Fictional characters wreck elementary logic, as they have contradictions and no excluded middle [Bostock] |
| 13623 | The syntactic turnstile |- φ means 'there is a proof of φ' or 'φ is a theorem' [Bostock] |
| 13347 | Validity is a conclusion following for premises, even if there is no proof [Bostock] |
| 13348 | It seems more natural to express |= as 'therefore', rather than 'entails' [Bostock] |
| 13349 | Γ|=φ is 'entails'; Γ|= is 'is inconsistent'; |=φ is 'valid' [Bostock] |
| 13614 | MPP: 'If Γ|=φ and Γ|=φ→ψ then Γ|=ψ' (omit Γs for Detachment) [Bostock] |
| 13617 | MPP is a converse of Deduction: If Γ |- φ→ψ then Γ,φ|-ψ [Bostock] |
| 13799 | The sign '=' is a two-place predicate expressing that 'a is the same thing as b' (a=b) [Bostock] |
| 13800 | |= α=α and α=β |= φ(α/ξ ↔ φ(β/ξ) fix identity [Bostock] |
| 13803 | If we are to express that there at least two things, we need identity [Bostock] |
| 13357 | Truth-functors are usually held to be defined by their truth-tables [Bostock] |
| 13812 | A 'zero-place' function just has a single value, so it is a name [Bostock] |
| 13811 | A 'total' function ranges over the whole domain, a 'partial' function over appropriate inputs [Bostock] |
| 13360 | In logic, a name is just any expression which refers to a particular single object [Bostock] |
| 13361 | An expression is only a name if it succeeds in referring to a real object [Bostock] |
| 13813 | Definite descriptions don't always pick out one thing, as in denials of existence, or errors [Bostock] |
| 13814 | Definite desciptions resemble names, but can't actually be names, if they don't always refer [Bostock] |
| 13816 | Because of scope problems, definite descriptions are best treated as quantifiers [Bostock] |
| 13817 | Definite descriptions are usually treated like names, and are just like them if they uniquely refer [Bostock] |
| 13848 | We are only obliged to treat definite descriptions as non-names if only the former have scope [Bostock] |
| 13815 | Names do not have scope problems (e.g. in placing negation), but Russell's account does have that problem [Bostock] |
| 13438 | 'Prenex normal form' is all quantifiers at the beginning, out of the scope of truth-functors [Bostock] |
| 13818 | If we allow empty domains, we must allow empty names [Bostock] |
| 13801 | An 'informal proof' is in no particular system, and uses obvious steps and some ordinary English [Bostock] |
| 13619 | Quantification adds two axiom-schemas and a new rule [Bostock] |
| 13622 | Axiom systems from Frege, Russell, Church, Lukasiewicz, Tarski, Nicod, Kleene, Quine... [Bostock] |
| 13615 | 'Conditonalised' inferences point to the Deduction Theorem: If Γ,φ|-ψ then Γ|-φ→ψ [Bostock] |
| 13616 | The Deduction Theorem greatly simplifies the search for proof [Bostock] |
| 13620 | Proof by Assumptions can always be reduced to Proof by Axioms, using the Deduction Theorem [Bostock] |
| 13621 | The Deduction Theorem and Reductio can 'discharge' assumptions - they aren't needed for the new truth [Bostock] |
| 13753 | Natural deduction takes proof from assumptions (with its rules) as basic, and axioms play no part [Bostock] |
| 13755 | Excluded middle is an introduction rule for negation, and ex falso quodlibet will eliminate it [Bostock] |
| 13758 | In natural deduction we work from the premisses and the conclusion, hoping to meet in the middle [Bostock] |
| 13754 | Natural deduction rules for → are the Deduction Theorem (→I) and Modus Ponens (→E) [Bostock] |
| 13611 | Tableau proofs use reduction - seeking an impossible consequence from an assumption [Bostock] |
| 13613 | A completed open branch gives an interpretation which verifies those formulae [Bostock] |
| 13612 | Non-branching rules add lines, and branching rules need a split; a branch with a contradiction is 'closed' [Bostock] |
| 13761 | In a tableau proof no sequence is established until the final branch is closed; hypotheses are explored [Bostock] |
| 13756 | A tree proof becomes too broad if its only rule is Modus Ponens [Bostock] |
| 13762 | Tableau rules are all elimination rules, gradually shortening formulae [Bostock] |
| 13757 | Unlike natural deduction, semantic tableaux have recipes for proving things [Bostock] |
| 13760 | A sequent calculus is good for comparing proof systems [Bostock] |
| 13759 | Each line of a sequent calculus is a conclusion of previous lines, each one explicitly recorded [Bostock] |
| 13364 | Interpretation by assigning objects to names, or assigning them to variables first [Bostock, by PG] |
| 13821 | Extensionality is built into ordinary logic semantics; names have objects, predicates have sets of objects [Bostock] |
| 13362 | If an object has two names, truth is undisturbed if the names are swapped; this is Extensionality [Bostock] |
| 13541 | For 'negation-consistent', there is never |-(S)φ and |-(S)¬φ [Bostock] |
| 13542 | A proof-system is 'absolutely consistent' iff we don't have |-(S)φ for every formula [Bostock] |
| 13540 | A set of formulae is 'inconsistent' when there is no interpretation which can make them all true [Bostock] |
| 13544 | Inconsistency or entailment just from functors and quantifiers is finitely based, if compact [Bostock] |
| 13618 | Compactness means an infinity of sequents on the left will add nothing new [Bostock] |
| 13358 | Ordinary or mathematical induction assumes for the first, then always for the next, and hence for all [Bostock] |
| 13359 | Complete induction assumes for all numbers less than n, then also for n, and hence for all numbers [Bostock] |
| 13543 | A relation is not reflexive, just because it is transitive and symmetrical [Bostock] |
| 13802 | Relations can be one-many (at most one on the left) or many-one (at most one on the right) [Bostock] |
| 13847 | If non-existent things are self-identical, they are just one thing - so call it the 'null object' [Bostock] |
| 13820 | The idea that anything which can be proved is necessary has a problem with empty names [Bostock] |
| 14858 | Being certain presumes that there are absolute truths, and means of arriving at them [Nietzsche] |
| 14830 | Intuition only recognises what is possible, not what exists or is certain [Nietzsche] |
| 20378 | Just as skin hides the horrors of the body, vanity conceals the passions of the soul [Nietzsche] |
| 13363 | A (modern) predicate is the result of leaving a gap for the name in a sentence [Bostock] |
| 14820 | People always do what they think is right, according to the degree of their intellect [Nietzsche] |
| 14856 | Our judgment seems to cause our nature, but actually judgment arises from our nature [Nietzsche] |
| 14842 | Why are the strong tastes of other people so contagious? [Nietzsche] |
| 14835 | Artists are not especially passionate, but they pretend to be [Nietzsche] |
| 22473 | Nietzsche said the will doesn't exist, so it can't ground moral responsibility [Nietzsche, by Foot] |
| 14807 | The history of morality rests on an error called 'responsibility', which rests on an error called 'free will' [Nietzsche] |
| 14823 | Ceasing to believe in human responsibility is bitter, if you had based the nobility of humanity on it [Nietzsche] |
| 14824 | It is absurd to blame nature and necessity; we should no more praise actions than we praise plants or artworks [Nietzsche] |
| 14812 | Intellect is tied to morality, because it requires good memory and powerful imagination [Nietzsche] |
| 14810 | Originally it was the rulers who requited good for good and evil for evil who were called 'good' [Nietzsche] |
| 14831 | No one has ever done anything that was entirely for other people [Nietzsche] |
| 14855 | Simultaneous love and respect are impossible; love has no separation or rank, but respect admits power [Nietzsche] |
| 14815 | We get enormous pleasure from tales of noble actions [Nietzsche] |
| 14849 | We can only achieve happy moments, not happy eras [Nietzsche] |
| 14818 | First morality is force, then custom, then acceptance, then instinct, then a pleasure - and finally 'virtue' [Nietzsche] |
| 20103 | You are mastered by your own virtues, but you must master them, and turn them into tools [Nietzsche] |
| 14817 | The 'good' man does the moral thing as if by nature, easily and gladly, after a long inheritance [Nietzsche] |
| 14809 | All societies of good men give a priority to gratitude [Nietzsche] |
| 14816 | Justice (fairness) originates among roughly equal powers (as the Melian dialogues show) [Nietzsche] |
| 20112 | Pity consoles those who suffer, because they see that they still have the power to hurt [Nietzsche] |
| 14821 | Apart from philosophers, most people rightly have a low estimate of pity [Nietzsche] |
| 14841 | Many people are better at having good friends than being a good friend [Nietzsche] |
| 14843 | Women can be friends with men, but only some physical antipathy will maintain it [Nietzsche] |
| 14811 | In Homer it is the contemptible person, not the harmful person, who is bad [Nietzsche] |
| 20111 | We could live more naturally, relishing the spectacle, and not thinking we are special [Nietzsche] |
| 14844 | People do not experience boredom if they have never learned to work properly [Nietzsche] |
| 14808 | Over huge periods of time human character would change endlessly [Nietzsche] |
| 14822 | If self-defence is moral, then so are most expressions of 'immoral' egoism [Nietzsche] |
| 14838 | The state aims to protect individuals from one another [Nietzsche] |
| 14852 | Culture cannot do without passions and vices [Nietzsche] |
| 14846 | If we want the good life for the greatest number, we must let them decide on the good life [Nietzsche] |
| 14819 | Slavery cannot be judged by our standards, because the sense of justice was then less developed [Nietzsche] |
| 14847 | Laws that are well thought out, or laws that are easy to understand? [Nietzsche] |
| 14814 | Execution is worse than murder, because we are using the victim, and really we are the guilty [Nietzsche] |
| 14836 | People will enthusiastically pursue an unwanted war, once sacrifices have been made [Nietzsche] |
| 14845 | Don't crush girls with dull Gymnasium education, the way we have crushed boys! [Nietzsche] |
| 14848 | Education in large states is mediocre, like cooking in large kitchens [Nietzsche] |
| 14839 | Interest in education gains strength when we lose interest in God [Nietzsche] |
| 14834 | Teachers only gather knowledge for their pupils, and can't be serious about themselves [Nietzsche] |
| 14825 | In religious thought nature is a complex of arbitrary acts by conscious beings [Nietzsche] |
| 14826 | Modern man wants laws of nature in order to submit to them [Nietzsche] |
| 14827 | The Greeks saw the gods not as their masters, but as idealised versions of themselves [Nietzsche] |
| 14813 | Science rejecting the teaching of Christianity in favour of Epicurus shows the superiority of the latter [Nietzsche] |
| 14832 | The Sermon on the Mount is vanity - praying to one part of oneself, and demonising the rest [Nietzsche] |
| 14850 | Christ was the noblest human being [Nietzsche] |
| 14837 | Christ seems warm hearted, and suppressed intellect in favour of the intellectually weak [Nietzsche] |
| 16713 | Philosophers are the forefathers of heretics [Tertullian] |
| 14828 | Religion is tempting if your life is boring, but you can't therefore impose it on the busy people [Nietzsche] |
| 6610 | I believe because it is absurd [Tertullian] |