80 ideas
2056 | Philosophers are always switching direction to something more interesting [Plato] |
2086 | Understanding mainly involves knowing the elements, not their combinations [Plato] |
2083 | Either a syllable is its letters (making parts as knowable as whole) or it isn't (meaning it has no parts) [Plato] |
2082 | A rational account is essentially a weaving together of things with names [Plato] |
2052 | Eristic discussion is aggressive, but dialectic aims to help one's companions in discussion [Plato] |
15854 | A primary element has only a name, and no logos, but complexes have an account, by weaving the names [Plato] |
13689 | 'Theorems' are formulas provable from no premises at all [Sider] |
13705 | Truth tables assume truth functionality, and are just pictures of truth functions [Sider] |
13706 | Intuitively, deontic accessibility seems not to be reflexive, but to be serial [Sider] |
13710 | In D we add that 'what is necessary is possible'; then tautologies are possible, and contradictions not necessary [Sider] |
13711 | System B introduces iterated modalities [Sider] |
13708 | S5 is the strongest system, since it has the most valid formulas, because it is easy to be S5-valid [Sider] |
13712 | Epistemic accessibility is reflexive, and allows positive and negative introspection (KK and K¬K) [Sider] |
13714 | We can treat modal worlds as different times [Sider] |
13720 | Converse Barcan Formula: □∀αφ→∀α□φ [Sider] |
13718 | The Barcan Formula ∀x□Fx→□∀xFx may be a defect in modal logic [Sider] |
13723 | System B is needed to prove the Barcan Formula [Sider] |
13715 | You can employ intuitionist logic without intuitionism about mathematics [Sider] |
13678 | The most popular account of logical consequence is the semantic or model-theoretic one [Sider] |
13679 | Maybe logical consequence is more a matter of provability than of truth-preservation [Sider] |
13682 | Maybe logical consequence is impossibility of the premises being true and the consequent false [Sider] |
13680 | Maybe logical consequence is a primitive notion [Sider] |
13722 | A 'theorem' is an axiom, or the last line of a legitimate proof [Sider] |
13696 | When a variable is 'free' of the quantifier, the result seems incapable of truth or falsity [Sider] |
13700 | A 'total' function must always produce an output for a given domain [Sider] |
13703 | λ can treat 'is cold and hungry' as a single predicate [Sider] |
13688 | Good axioms should be indisputable logical truths [Sider] |
13687 | No assumptions in axiomatic proofs, so no conditional proof or reductio [Sider] |
13690 | Proof by induction 'on the length of the formula' deconstructs a formula into its accepted atoms [Sider] |
13691 | Induction has a 'base case', then an 'inductive hypothesis', and then the 'inductive step' [Sider] |
13685 | Natural deduction helpfully allows reasoning with assumptions [Sider] |
13686 | We can build proofs just from conclusions, rather than from plain formulae [Sider] |
13697 | Valuations in PC assign truth values to formulas relative to variable assignments [Sider] |
13684 | The semantical notion of a logical truth is validity, being true in all interpretations [Sider] |
13704 | It is hard to say which are the logical truths in modal logic, especially for iterated modal operators [Sider] |
13724 | In model theory, first define truth, then validity as truth in all models, and consequence as truth-preservation [Sider] |
13698 | In a complete logic you can avoid axiomatic proofs, by using models to show consequences [Sider] |
13699 | Compactness surprisingly says that no contradictions can emerge when the set goes infinite [Sider] |
13701 | A single second-order sentence validates all of arithmetic - but this can't be proved axiomatically [Sider] |
10216 | We master arithmetic by knowing all the numbers in our soul [Plato] |
2060 | There seem to be two sorts of change: alteration and motion [Plato] |
13692 | A 'precisification' of a trivalent interpretation reduces it to a bivalent interpretation [Sider] |
13695 | Supervaluational logic is classical, except when it adds the 'Definitely' operator [Sider] |
13693 | A 'supervaluation' assigns further Ts and Fs, if they have been assigned in every precisification [Sider] |
13694 | We can 'sharpen' vague terms, and then define truth as true-on-all-sharpenings [Sider] |
13683 | A relation is a feature of multiple objects taken together [Sider] |
2084 | If a word has no parts and has a single identity, it turns out to be the same kind of thing as a letter [Plato] |
15844 | A sum is that from which nothing is lacking, which is a whole [Plato] |
15843 | The whole can't be the parts, because it would be all of the parts, which is the whole [Plato] |
13702 | The identity of indiscernibles is necessarily true, if being a member of some set counts as a property [Sider] |
13721 | 'Strong' necessity in all possible worlds; 'weak' necessity in the worlds where the relevant objects exist [Sider] |
13707 | Maybe metaphysical accessibility is intransitive, if a world in which I am a frog is impossible [Sider] |
13709 | Logical truths must be necessary if anything is [Sider] |
13716 | 'If B hadn't shot L someone else would have' if false; 'If B didn't shoot L, someone else did' is true [Sider] |
13717 | Transworld identity is not a problem in de dicto sentences, which needn't identify an individual [Sider] |
13719 | Barcan Formula problem: there might have been a ghost, despite nothing existing which could be a ghost [Sider] |
2080 | Things are only knowable if a rational account (logos) is possible [Plato] |
16126 | Expertise is knowledge of the whole by means of the parts [Plato] |
2050 | It is impossible to believe something which is held to be false [Plato] |
2076 | How can a belief exist if its object doesn't exist? [Plato] |
2045 | Perception is infallible, suggesting that it is knowledge [Plato] |
2067 | Our senses could have been separate, but they converge on one mind [Plato] |
2068 | With what physical faculty do we perceive pairs of opposed abstract qualities? [Plato] |
2078 | You might mistake eleven for twelve in your senses, but not in your mind [Plato] |
2069 | Thought must grasp being itself before truth becomes possible [Plato] |
2089 | An inadequate rational account would still not justify knowledge [Plato] |
2085 | Parts and wholes are either equally knowable or equally unknowable [Plato] |
2091 | Without distinguishing marks, how do I know what my beliefs are about? [Plato] |
2087 | A rational account might be seeing an image of one's belief, like a reflection in a mirror [Plato] |
2090 | A rational account involves giving an image, or analysis, or giving a differentiating mark [Plato] |
2081 | Maybe primary elements can be named, but not receive a rational account [Plato] |
2088 | A rational account of a wagon would mean knowledge of its hundred parts [Plato] |
2047 | What evidence can be brought to show whether we are dreaming or not? [Plato] |
2053 | If you claim that all beliefs are true, that includes beliefs opposed to your own [Plato] |
2059 | How can a relativist form opinions about what will happen in the future? [Plato] |
2054 | Clearly some people are superior to others when it comes to medicine [Plato] |
20890 | Why do sexual relationships need permanence, if other relationships don't? [Punzo] |
20891 | Does engaging in sexual intercourse really need no more thought than playing tennis? [Punzo] |
2058 | God must be the epitome of goodness, and we can only approach a divine state by being as good as possible [Plato] |
2057 | There must always be some force of evil ranged against good [Plato] |