31 ideas
| 20109 | Hegel inserted society and history between the God-world, man-nature, man-being binary pairs [Hegel, by Safranski] |
| Full Idea: Before Hegel, people thought in binary oppositions of God and the world, man and nature, man and being. After Hegel an intervening world of society and history was inserted between these pairs. | |
| From: report of Georg W.F.Hegel (Introduction to the Philosophy of History [1840]) by Rüdiger Safranski - Nietzsche: a philosophical biography 05 | |
| A reaction: This is what Popper later called 'World Three'. This might be seen as the start of what we islanders call 'continental' philosophy, which we have largely ignored. Analytic philosophy only discovered this through philosophy of language. |
| 17749 | Post proved the consistency of propositional logic in 1921 [Walicki] |
| Full Idea: A proof of the consistency of propositional logic was given by Emil Post in 1921. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], History E.2.1) |
| 17765 | Propositional language can only relate statements as the same or as different [Walicki] |
| Full Idea: Propositional language is very rudimentary and has limited powers of expression. The only relation between various statements it can handle is that of identity and difference. As are all the same, but Bs can be different from As. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 7 Intro) | |
| A reaction: [second sentence a paraphrase] In predicate logic you could represent two statements as being the same except for one element (an object or predicate or relation or quantifier). |
| 17764 | Boolean connectives are interpreted as functions on the set {1,0} [Walicki] |
| Full Idea: Boolean connectives are interpreted as functions on the set {1,0}. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 5.1) | |
| A reaction: 1 and 0 are normally taken to be true (T) and false (F). Thus the functions output various combinations of true and false, which are truth tables. |
| 17752 | The empty set is useful for defining sets by properties, when the members are not yet known [Walicki] |
| Full Idea: The empty set is mainly a mathematical convenience - defining a set by describing the properties of its members in an involved way, we may not know from the very beginning what its members are. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 1.1) |
| 17753 | The empty set avoids having to take special precautions in case members vanish [Walicki] |
| Full Idea: Without the assumption of the empty set, one would often have to take special precautions for the case where a set happened to contain no elements. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 1.1) | |
| A reaction: Compare the introduction of the concept 'zero', where special precautions are therefore required. ...But other special precautions are needed without zero. Either he pays us, or we pay him, or ...er. Intersecting sets need the empty set. |
| 17759 | Ordinals play the central role in set theory, providing the model of well-ordering [Walicki] |
| Full Idea: Ordinals play the central role in set theory, providing the paradigmatic well-orderings. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.3) | |
| A reaction: When you draw the big V of the iterative hierarchy of sets (built from successive power sets), the ordinals are marked as a single line up the middle, one ordinal for each level. |
| 17741 | To determine the patterns in logic, one must identify its 'building blocks' [Walicki] |
| Full Idea: In order to construct precise and valid patterns of arguments one has to determine their 'building blocks'. One has to identify the basic terms, their kinds and means of combination. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], History Intro) | |
| A reaction: A deceptively simple and important idea. All explanation requires patterns and levels, and it is the idea of building blocks which makes such things possible. It is right at the centre of our grasp of everything. |
| 17747 | A 'model' of a theory specifies interpreting a language in a domain to make all theorems true [Walicki] |
| Full Idea: A specification of a domain of objects, and of the rules for interpreting the symbols of a logical language in this domain such that all the theorems of the logical theory are true is said to be a 'model' of the theory. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], History E.1.3) | |
| A reaction: The basic ideas of this emerged 1915-30, but it needed Tarski's account of truth to really get it going. |
| 17748 | The L-S Theorem says no theory (even of reals) says more than a natural number theory [Walicki] |
| Full Idea: The L-S Theorem is ...a shocking result, since it implies that any consistent formal theory of everything - even about biology, physics, sets or the real numbers - can just as well be understood as being about natural numbers. It says nothing more. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], History E.2) | |
| A reaction: Illuminating. Particularly the point that no theory about the real numbers can say anything more than a theory about the natural numbers. So the natural numbers contain all the truths we can ever express? Eh????? |
| 17761 | A compact axiomatisation makes it possible to understand a field as a whole [Walicki] |
| Full Idea: Having such a compact [axiomatic] presentation of a complicated field [such as Euclid's], makes it possible to relate not only to particular theorems but also to the whole field as such. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 4.1) |
| 17763 | Axiomatic systems are purely syntactic, and do not presuppose any interpretation [Walicki] |
| Full Idea: Axiomatic systems, their primitive terms and proofs, are purely syntactic, that is, do not presuppose any interpretation. ...[142] They never address the world directly, but address a possible semantic model which formally represents the world. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 4.1) |
| 17758 | Ordinals are transitive sets of transitive sets; or transitive sets totally ordered by inclusion [Walicki] |
| Full Idea: An ordinal can be defined as a transitive set of transitive sets, or else, as a transitive set totally ordered by set inclusion. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.3) |
| 17755 | Ordinals are the empty set, union with the singleton, and any arbitrary union of ordinals [Walicki] |
| Full Idea: The collection of ordinals is defined inductively: Basis: the empty set is an ordinal; Ind: for an ordinal x, the union with its singleton is also an ordinal; and any arbitrary (possibly infinite) union of ordinals is an ordinal. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.3) | |
| A reaction: [symbolism translated into English] Walicki says they are called 'ordinal numbers', but are in fact a set. |
| 17756 | The union of finite ordinals is the first 'limit ordinal'; 2ω is the second... [Walicki] |
| Full Idea: We can form infinite ordinals by taking unions of ordinals. We can thus form 'limit ordinals', which have no immediate predecessor. ω is the first (the union of all finite ordinals), ω + ω = sω is second, 3ω the third.... | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.3) |
| 17760 | Two infinite ordinals can represent a single infinite cardinal [Walicki] |
| Full Idea: There may be several ordinals for the same cardinality. ...Two ordinals can represent different ways of well-ordering the same number (aleph-0) of elements. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.3) | |
| A reaction: This only applies to infinite ordinals and cardinals. For the finite, the two coincide. In infinite arithmetic the rules are different. |
| 17757 | Members of ordinals are ordinals, and also subsets of ordinals [Walicki] |
| Full Idea: Every member of an ordinal is itself an ordinal, and every ordinal is a transitive set (its members are also its subsets; a member of a member of an ordinal is also a member of the ordinal). | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.3) |
| 17762 | In non-Euclidean geometry, all Euclidean theorems are valid that avoid the fifth postulate [Walicki] |
| Full Idea: Since non-Euclidean geometry preserves all Euclid's postulates except the fifth one, all the theorems derived without the use of the fifth postulate remain valid. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 4.1) |
| 17754 | Inductive proof depends on the choice of the ordering [Walicki] |
| Full Idea: Inductive proof is not guaranteed to work in all cases and, particularly, it depends heavily on the choice of the ordering. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], 2.1.1) | |
| A reaction: There has to be an well-founded ordering for inductive proofs to be possible. |
| 17742 | Scotus based modality on semantic consistency, instead of on what the future could allow [Walicki] |
| Full Idea: The link between time and modality was severed by Duns Scotus, who proposed a notion of possibility based purely on the notion of semantic consistency. 'Possible' means for him logically possible, that is, not involving contradiction. | |
| From: Michal Walicki (Introduction to Mathematical Logic [2012], History B.4) |
| 8808 | Involuntary beliefs can still be evaluated [Feldman/Conee] |
| Full Idea: Examples confirm that beliefs may be both involuntary and subject to epistemic evaluation. | |
| From: R Feldman / E Conee (Evidentialism [1985], II) | |
| A reaction: This is an extremely important point, which summarises the situation with beliefs that arise from (apparent) immediate perception. A belief cannot possibly be knowledge if it has been triggered, but no effort was made to evaluate it. |
| 8807 | Evidentialism is the view that justification is determined by the quality of the evidence [Feldman/Conee] |
| Full Idea: What we call 'evidentialism' is the view that the epistemic justification of a belief is determined by the quality of the believer's evidence for the belief. | |
| From: R Feldman / E Conee (Evidentialism [1985], I) | |
| A reaction: The immediate question is whether the believer knows the quality of their evidence. A detective might not recognise the crucial clue (like the dog not barking). The definition of 'quality' had better not turn out to be circular. Forgotten evidence? |
| 8809 | Beliefs should fit evidence, and if you ought to believe it, then you are justified [Feldman/Conee] |
| Full Idea: One epistemically ought to have the doxastic attitudes that fit one's evidence. Being epistemically obligatory is equivalent to being epistemically justified. | |
| From: R Feldman / E Conee (Evidentialism [1985], III) | |
| A reaction: It is normal for someone to refuse to accept something, when another person believes the evidence is overwhelming. Evaluation of evidence must include an assessment of what other evidence might turn up. |
| 8810 | If someone rejects good criticism through arrogance, that is irrelevant to whether they have knowledge [Feldman/Conee] |
| Full Idea: If an arrogant young physicist refuses to recognise valid criticisms from a senior colleague, his or her character has nothing to do with the epistemic status of their belief in the theory. | |
| From: R Feldman / E Conee (Evidentialism [1985], III) | |
| A reaction: This rejects the idea that epistemic justification is essentially a matter of virtues and vices of character. That view is a version of reliabilism, and hence of externalism. I agree with the criticism, but epistemic virtues are still significant. |
| 23274 | World history has no room for happiness [Hegel] |
| Full Idea: World history is not the place for happiness. Periods of happiness are empty pages in history. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 3) | |
| A reaction: Clearly, Hegel thinks the progress of world history is much more important than happiness. This idea gives backing to those who don't care much about the casualties on either side in a major war. |
| 23275 | The state of nature is one of untamed brutality [Hegel] |
| Full Idea: The 'state of nature' is not an ideal condition, but a condition of injustice, of violence, of untamed natural drives, inhuman acts and emotions. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 3) | |
| A reaction: He agrees with Hobbes, and disagrees with Rousseau. Hobbes's solution is authoritarian monarchy, but Hegel's solution is the unified and focused state, in which freedom can be realised. |
| 23276 | The soul of the people is an organisation of its members which produces an essential unity [Hegel] |
| Full Idea: The soul [of the people] exists only insofar as it is an organisation of its members, which - by taking itself together in its simple unity - produce the soul. Thus the people is one individuality in its essence. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 3) | |
| A reaction: Hegel is seen (e.g. by Charles Taylor) as the ancestor of a rather attractive communitarianism, but I think Popper is more accurate in seeing him as the first stage of modern totalitarianism. The people seen as one individual terrifies me. |
| 23272 | The human race matters, and individuals have little importance [Hegel] |
| Full Idea: Individuals are of slight importance compared to the mass of the human race. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 3) | |
| A reaction: A perfect statement of the anti-liberal viewpoint. Hegel is complex, but this is the strand that leads to ridiculous totalitarianism, where the highest ideal is to die for the glory of your nation. Importance can only start from individuals. |
| 23273 | In a good state the goal of the citizens and of the whole state are united [Hegel] |
| Full Idea: A state is well constituted and internally strong if the private interest of the citizens is united in the universal goal of the state. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 3) | |
| A reaction: The obvious question is who decides on the goals, and what to do with the citizens who don't accept them. |
| 23271 | The goal of the world is Spirit's consciousness and enactment of freedom [Hegel] |
| Full Idea: The final goal of the world is Spirit's consciousness of its freedom, and hence also the actualisation of that very freedom. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 3) | |
| A reaction: I have the impression that this ridiculous idea has been very influential in modern French philosophy, since they all seem to be dreaming of some perfect freedom at the end of the rainbow. Freedom is good, but this gives it a bad name. |
| 23270 | We should all agree that there is reason in history [Hegel] |
| Full Idea: We ought to have the firm and unconquerable belief that there is reason in history. | |
| From: Georg W.F.Hegel (Introduction to the Philosophy of History [1840], 2) | |
| A reaction: This is a ridiculous but hugely influential idea, and I have no idea what makes Hegel believe it. It is the Stoic idea that nature is intrinsically rational, but extending it to human history is absurd. Human exceptionalism. Needs a dose of Darwin. |