104 ideas
| 5182 | Claims about 'the Absolute' are not even verifiable in principle [Ayer on Bradley] |
| 6864 | Metaphysics is finding bad reasons for instinctive beliefs [Bradley] |
| 19767 | Reason leads to prudent selfishness, which overrules natural compassion [Rousseau] |
| 9535 | 'Contradictory' propositions always differ in truth-value [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] |
| 9514 | If A and B are 'interderivable' from one another we may write A -||- B [Lemmon] |
| 9511 | We write the conditional 'if P (antecedent) then Q (consequent)' as P→Q [Lemmon] |
| 9512 | We write the 'negation' of P (not-P) as ¬ [Lemmon] |
| 9510 | That proposition that either P or Q is their 'disjunction', written P∨Q [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] |
| 9516 | A 'well-formed formula' follows the rules for variables, ¬, →, ∧, ∨, and ↔ [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] |
| 9534 | Two propositions are 'equivalent' if they mirror one another's truth-value [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] |
| 9533 | A 'implies' B if B is true whenever A is true (so that A→B is tautologous) [Lemmon] |
| 9532 | 'Subcontrary' propositions are never both false, so that A∨B is a tautology [Lemmon] |
| 9531 | 'Contrary' propositions are never both true, 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] |
| 9530 | A wff is 'contingent' if produces at least one T and at least one F [Lemmon] |
| 9396 | DN: Given A, we may derive ¬¬A [Lemmon] |
| 9398 | ∧I: Given A and B, we may derive A∧B [Lemmon] |
| 9394 | MPP: Given A and A→B, we may derive B [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] |
| 9395 | MTT: Given ¬B and A→B, we derive ¬A [Lemmon] |
| 9393 | A: we may assume any proposition at any stage [Lemmon] |
| 9400 | ∨I: Given either A or B separately, we may derive A∨B [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] |
| 9522 | 'Modus ponendo tollens' (MPT) says P, ¬(P ∧ Q) |- ¬Q [Lemmon] |
| 9526 | We can change conjunctions into negated conditionals with P→Q -||- ¬(P → ¬Q) [Lemmon] |
| 9527 | The Distributive Laws can rearrange a pair of conjunctions or disjunctions [Lemmon] |
| 9523 | De Morgan's Laws make negated conjunctions/disjunctions into non-negated disjunctions/conjunctions [Lemmon] |
| 9524 | We can change conditionals into disjunctions with P→Q -||- ¬P ∨ Q [Lemmon] |
| 9525 | We can change conditionals into negated conjunctions with P→Q -||- ¬(P ∧ ¬Q) [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] |
| 10999 | Names need a means of reidentifying their referents [Bradley, by Read] |
| 6422 | Internal relations are said to be intrinsic properties of two terms, and of the whole they compose [Bradley, by Russell] |
| 7966 | Relations must be linked to their qualities, but that implies an infinite regress of relations [Bradley] |
| 19757 | No one would bother to reason, and try to know things, without a desire for enjoyment [Rousseau] |
| 6404 | British Idealists said reality is a single Mind which experiences itself [Bradley, by Grayling] |
| 22299 | Bradley's objective idealism accepts reality (the Absolute), but says we can't fully describe it [Bradley, by Potter] |
| 21343 | Qualities and relations are mere appearance; the Absolute is a single undifferentiated substance [Bradley, by Heil] |
| 19760 | General ideas are purely intellectual; imagining them is immediately particular [Rousseau] |
| 19759 | Only words can introduce general ideas into the mind [Rousseau] |
| 19758 | Language may aid thinking, but powerful thought was needed to produce language [Rousseau] |
| 19773 | Without love, what use is beauty? [Rousseau] |
| 19769 | Rational morality is OK for brainy people, but ordinary life can't rely on that [Rousseau] |
| 19752 | If we should not mistreat humans, it is mainly because of sentience, not rationality [Rousseau] |
| 19768 | The better Golden Rule is 'do good for yourself without harming others' [Rousseau] |
| 19766 | The fact that we weep (e.g. in theatres) shows that we are naturally compassionate [Rousseau] |
| 19756 | Humans are less distinguished from other animals by understanding, than by being free agents [Rousseau] |
| 19755 | Most human ills are self-inflicted; the simple, solitary, regular natural life is good [Rousseau] |
| 19762 | Is language a pre-requisite for society, or might it emerge afterwards? [Rousseau] |
| 19763 | I doubt whether a savage person ever complains of life, or considers suicide [Rousseau] |
| 19765 | Savages avoid evil because they are calm, and never think of it (not because they know goodness) [Rousseau] |
| 19779 | Primitive man was very gentle [Rousseau] |
| 19771 | Savage men quietly pursue desires, without the havoc of modern frenzied imagination [Rousseau] |
| 19778 | Leisure led to envy, inequality, vice and revenge, which we now see in savages [Rousseau] |
| 19751 | Our two starting principles are concern for self-interest, and compassion for others [Rousseau] |
| 19774 | A savage can steal fruit or a home, but there is no means of achieving obedience [Rousseau] |
| 19772 | In a state of nature people are much more equal; it is society which increases inequalities [Rousseau] |
| 19789 | It is against nature for children to rule old men, fools to rule the wise, and the rich to hog resources [Rousseau] |
| 19787 | People accept the right to be commanded, because they themselves wish to command [Rousseau] |
| 19780 | We seem to have made individual progress since savagery, but actually the species has decayed [Rousseau] |
| 19747 | Revolutionaries usually confuse liberty with total freedom, and end up with heavier chains [Rousseau] |
| 19748 | Plebiscites are bad, because they exclude the leaders from crucial decisions [Rousseau] |
| 19749 | In a direct democracy, only the leaders should be able to propose new laws [Rousseau] |
| 19775 | People must be made dependent before they can be enslaved [Rousseau] |
| 19784 | Enslaved peoples often boast of their condition, calling it a state of 'peace' [Rousseau] |
| 19785 | If the child of a slave woman is born a slave, then a man is not born a man [Rousseau] |
| 19746 | Like rich food, liberty can ruin people who are too weak to cope with it [Rousseau] |
| 19786 | Three stages of the state produce inequalities of wealth, power, and enslavement [Rousseau] |
| 19788 | The pleasure of wealth and power is largely seeing others deprived of them [Rousseau] |
| 19777 | Persuading other people that some land was 'owned' was the beginning of society [Rousseau] |
| 19782 | What else could property arise from, but the labour people add to it? [Rousseau] |
| 19781 | Land cultivation led to a general right of ownership, administered justly [Rousseau] |
| 19754 | If we have a natural right to property, what exactly does 'belonging to' mean? [Rousseau] |
| 19750 | Writers just propose natural law as the likely useful agreements among people [Rousseau] |
| 19770 | Primitive people simply redressed the evil caused by violence, without thought of punishing [Rousseau] |
| 19783 | A state of war remains after a conquest, if the losers don't accept the winners [Rousseau] |
| 19753 | Both men and animals are sentient, which should give the latter the right not to be mistreated [Rousseau] |
| 6406 | Reality is one, because plurality implies relations, and they assert a superior unity [Bradley] |
| 19761 | Men started with too few particular names, but later had too few natural kind names [Rousseau] |
| 19776 | Small uninterrupted causes can have big effects [Rousseau] |