106 ideas
| 2512 | Philosophy is a battle against the bewitchment of our intelligence by means of language [Wittgenstein] |
| 4148 | What is your aim in philosophy? - To show the fly the way out of the fly-bottle [Wittgenstein] |
| 22490 | Bring words back from metaphysics to everyday use [Wittgenstein] |
| 6566 | The problem is to explain the role of contradiction in social life [Wittgenstein] |
| 9535 | 'Contradictory' propositions always differ in truth-value [Lemmon] |
| 9511 | We write the conditional 'if P (antecedent) then Q (consequent)' as P→Q [Lemmon] |
| 9510 | That proposition that either P or Q is their 'disjunction', written P∨Q [Lemmon] |
| 9512 | We write the 'negation' of P (not-P) as ¬ [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] |
| 9514 | If A and B are 'interderivable' from one another we may write A -||- B [Lemmon] |
| 9509 | That proposition that both P and Q is their 'conjunction', written P∧Q [Lemmon] |
| 9508 | The sign |- may be read as 'therefore' [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] |
| 9519 | A 'substitution-instance' is a wff formed by consistent replacing variables with wffs [Lemmon] |
| 9529 | A wff is 'inconsistent' if all assignments to variables result in the value F [Lemmon] |
| 9531 | 'Contrary' propositions are never both true, so that ¬(A∧B) is a tautology [Lemmon] |
| 9534 | Two propositions are 'equivalent' if they mirror one another's truth-value [Lemmon] |
| 9530 | A wff is 'contingent' if produces at least one T and at least one F [Lemmon] |
| 9532 | 'Subcontrary' propositions are never both false, so that A∨B is a tautology [Lemmon] |
| 9533 | A 'implies' B if B is true whenever A is true (so that A→B is tautologous) [Lemmon] |
| 9528 | A wff is a 'tautology' if all assignments to variables result in the value T [Lemmon] |
| 9518 | A 'theorem' is the conclusion of a provable sequent with zero assumptions [Lemmon] |
| 9398 | ∧I: Given A and B, we may derive A∧B [Lemmon] |
| 9397 | CP: Given a proof of B from A as assumption, we may derive A→B [Lemmon] |
| 9394 | MPP: Given A and A→B, we may derive B [Lemmon] |
| 9402 | RAA: If assuming A will prove B∧¬B, then derive ¬A [Lemmon] |
| 9395 | MTT: Given ¬B and A→B, we derive ¬A [Lemmon] |
| 9400 | ∨I: Given either A or B separately, we may derive A∨B [Lemmon] |
| 9401 | ∨E: Derive C from A∨B, if C can be derived both from A and from B [Lemmon] |
| 9396 | DN: Given A, we may derive ¬¬A [Lemmon] |
| 9393 | A: we may assume any proposition at any stage [Lemmon] |
| 9399 | ∧E: Given A∧B, we may derive either A or B separately [Lemmon] |
| 9521 | 'Modus tollendo ponens' (MTP) says ¬P, P ∨ Q |- Q [Lemmon] |
| 9522 | 'Modus ponendo tollens' (MPT) says P, ¬(P ∧ Q) |- ¬Q [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] |
| 9523 | De Morgan's Laws make negated conjunctions/disjunctions into non-negated disjunctions/conjunctions [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] |
| 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] |
| 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] |
| 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] |
| 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] |
| 18743 | Wittgenstein says we want the grammar of problems, not their first-order logical structure [Wittgenstein, by Horsten/Pettigrew] |
| 4139 | Naming is a preparation for description [Wittgenstein] |
| 4946 | A name is not determined by a description, but by a cluster or family [Wittgenstein, by Kripke] |
| 15106 | Essence is expressed by grammar [Wittgenstein] |
| 6600 | The belief that fire burns is like the fear that it burns [Wittgenstein] |
| 4153 | Are sense-data the material of which the universe is made? [Wittgenstein] |
| 6501 | As sense-data are necessarily private, they are attacked by Wittgenstein's objections [Wittgenstein, by Robinson,H] |
| 11079 | How do I decide when to accept or obey an intuition? [Wittgenstein] |
| 4160 | One can mistrust one's own senses, but not one's own beliefs [Wittgenstein] |
| 19273 | I don't have the opinion that people have minds; I just treat them as such [Wittgenstein] |
| 5663 | It is irresponsible to generalise from my own case of pain to other people's [Wittgenstein] |
| 19272 | To imagine another's pain by my own, I must imagine a pain I don't feel, by one I do feel [Wittgenstein] |
| 4161 | If a lion could talk, we could not understand him [Wittgenstein] |
| 7392 | If a lion could talk, it would be nothing like other lions [Dennett on Wittgenstein] |
| 5210 | We could know what a lion thinks by mapping both its brain patterns and its experiences [Douglas,A] |
| 5676 | To say that I 'know' I am in pain means nothing more than that I AM in pain [Wittgenstein] |
| 4154 | Why are we not aware of the huge gap between mind and brain in ordinary life? [Wittgenstein] |
| 4158 | An 'inner process' stands in need of outward criteria [Wittgenstein] |
| 6165 | Every course of action can either accord or conflict with a rule, so there is no accord or conflict [Wittgenstein] |
| 4143 | One cannot obey a rule 'privately', because that is a practice, not the same as thinking one is obeying [Wittgenstein] |
| 7092 | If individuals can't tell if they are following a rule, how does a community do it? [Grayling on Wittgenstein] |
| 4138 | Is white simple, or does it consist of the colours of the rainbow? [Wittgenstein] |
| 7055 | Externalist accounts of mental content begin in Wittgenstein [Wittgenstein, by Heil] |
| 12576 | Possessing a concept is knowing how to go on [Wittgenstein, by Peacocke] |
| 4157 | Concepts direct our interests and investigations, and express those interests [Wittgenstein] |
| 12606 | Man learns the concept of the past by remembering [Wittgenstein] |
| 4141 | Various games have a 'family resemblance', as their similarities overlap and criss-cross [Wittgenstein] |
| 23450 | Wittgenstein rejected his earlier view that the form of language is the form of the world [Wittgenstein, by Morris,M] |
| 4150 | Asking about verification is only one way of asking about the meaning of a proposition [Wittgenstein] |
| 6567 | For Wittgenstein, words are defined by their use, just as chess pieces are [Wittgenstein, by Fogelin] |
| 6169 | We do not achieve meaning and understanding in our heads, but in the world [Wittgenstein, by Rowlands] |
| 4155 | We all seem able to see quite clearly how sentences represent things when we use them [Wittgenstein] |
| 4137 | In the majority of cases the meaning of a word is its use in the language [Wittgenstein] |
| 4142 | To understand a sentence means to understand a language [Wittgenstein] |
| 4149 | We don't have 'meanings' in our minds in addition to verbal expressions [Wittgenstein] |
| 4156 | Make the following experiment: say "It's cold here" and mean "It's warm here" [Wittgenstein] |
| 4145 | How do words refer to sensations? [Wittgenstein] |
| 4140 | The standard metre in Paris is neither one metre long nor not one metre long [Wittgenstein] |
| 6166 | Was Wittgenstein's problem between individual and community, or between occasions for an individual? [Rowlands on Wittgenstein] |
| 7875 | If a brilliant child invented a name for a private sensation, it couldn't communicate it [Wittgenstein] |
| 4146 | We cannot doublecheck mental images for correctness (or confirm news with many copies of the paper) [Wittgenstein] |
| 4147 | If we only named pain by our own case, it would be like naming beetles by looking in a private box [Wittgenstein] |
| 5659 | If the reference is private, that is incompatible with the sense being public [Wittgenstein, by Scruton] |
| 4152 | Getting from perceptions to words cannot be a private matter; the rules need an institution of use [Wittgenstein] |
| 4136 | To imagine a language means to imagine a form of life [Wittgenstein] |
| 4144 | Common human behaviour enables us to interpret an unknown language [Wittgenstein] |
| 11049 | To communicate, language needs agreement in judgment as well as definition [Wittgenstein] |
| 6658 | What is left over if I subtract my arm going up from my raising my arm? [Wittgenstein] |
| 4151 | Grammar tells what kind of object anything is - and theology is a kind of grammar [Wittgenstein] |
| 4159 | The human body is the best picture of the human soul [Wittgenstein] |