97 ideas
21405 | Cicero sees wisdom in terms of knowledge, but earlier Stoics saw it as moral [Cicero, by Long] |
20871 | Unfortunately we choose a way of life before we are old enough to think clearly [Cicero] |
5893 | A wise man has integrity, firmness of will, nobility, consistency, sobriety, patience [Cicero] |
5891 | Philosophy is the collection of rational arguments [Cicero] |
2661 | Dialectic is speech cast in the form of logical argument [Cicero] |
2653 | If the parts of the universe are subject to the law of nature, the whole universe must also be subject to it [Cicero] |
2673 | There cannot be more than one truth [Cicero] |
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] |
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] |
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] |
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] |
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] |
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] |
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] |
21677 | How can the not-true fail to be false, or the not-false fail to be true? [Cicero] |
2669 | Dialectic assumes that all statements are either true or false, but self-referential paradoxes are a big problem [Cicero] |
2664 | If we have complete healthy senses, what more could the gods give us? [Cicero] |
2665 | How can there be a memory of what is false? [Cicero] |
3061 | Anaxarchus said that he was not even sure that he knew nothing [Anaxarchus, by Diog. Laertius] |
20800 | Every true presentation can have a false one of the same quality [Cicero] |
5879 | The soul is the heart, or blood in the heart, or part of the brain, of something living in heart or brain, or breath [Cicero] |
5884 | How can one mind perceive so many dissimilar sensations? [Cicero] |
5887 | The soul has a single nature, so it cannot be divided, and hence it cannot perish [Cicero] |
5886 | Like the eye, the soul has no power to see itself, but sees other things [Cicero] |
6029 | Whoever knows future causes knows everything that will be [Cicero] |
2628 | Why would mind mix with matter if it didn't need it? [Cicero] |
5885 | Souls contain no properties of elements, and elements contain no properties of souls [Cicero] |
21667 | Oratory and philosophy are closely allied; orators borrow from philosophy, and ornament it [Cicero] |
20814 | Eloquence educates, exhorts, comforts, distracts and unites us, and raises us from savagery [Cicero] |
21678 | If desire is not in our power then neither are choices, so we should not be praised or punished [Cicero] |
2672 | Virtues must be very detached, to avoid being motivated by pleasure [Cicero] |
5890 | We should not share the distress of others, but simply try to relieve it [Cicero] |
5894 | All men except philosophers fear poverty [Cicero] |
6031 | The essence of propriety is consistency [Cicero] |
5895 | If one despises illiterate mechanics individually, they are not worth more collectively [Cicero] |
2640 | We have the death penalty, but still have thousands of robbers [Cicero] |
2652 | Some regard nature simply as an irrational force that imparts movement [Cicero] |
2645 | Why shouldn't the gods fear their own destruction? [Cicero] |
2627 | I wonder whether loss of reverence for the gods would mean the end of all virtue [Cicero] |
2651 | God doesn't obey the laws of nature; they are subject to the law of God [Cicero] |
2634 | It seems clear to me that we have an innate idea of the divine [Cicero] |
2636 | Many primitive people know nothing of the gods [Cicero] |
2655 | If the barbarians of Britain saw a complex machine, they would be baffled, but would know it was designed [Cicero] |
2656 | Chance is no more likely to create the world than spilling lots of letters is likely to create a famous poem [Cicero] |
2650 | If a person cannot feel the power of God when looking at the stars, they are probably incapable of feeling [Cicero] |
2647 | It is obvious from order that someone is in charge, as when we visit a gymnasium [Cicero] |
2657 | If everything with regular movement and order is divine, then recurrent illnesses must be divine [Cicero] |
2638 | Either the gods are identical, or one is more beautiful than another [Cicero] |
2635 | The gods are happy, so virtuous, so rational, so must have human shape [Cicero] |
2641 | Why believe in gods if you have never seen them? [Cicero] |
2659 | The lists of good men who have suffered and bad men who have prospered are endless [Cicero] |
2658 | The gods blame men for having vices, but they could have given us enough reason to avoid them [Cicero] |