18 ideas
| 13201 | ∈ says the whole set is in the other; ⊆ says the members of the subset are in the other [Enderton] |
| Full Idea: To know if A ∈ B, we look at the set A as a single object, and check if it is among B's members. But if we want to know whether A ⊆ B then we must open up set A and check whether its various members are among the members of B. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 1:04) | |
| A reaction: This idea is one of the key ideas to grasp if you are going to get the hang of set theory. John ∈ USA ∈ UN, but John is not a member of the UN, because he isn't a country. See Idea 12337 for a special case. |
| 13204 | The 'ordered pair' <x,y> is defined to be {{x}, {x,y}} [Enderton] |
| Full Idea: The 'ordered pair' <x,y> is defined to be {{x}, {x,y}}; hence it can be proved that <u,v> = <x,y> iff u = x and v = y (given by Kuratowski in 1921). ...The definition is somewhat arbitrary, and others could be used. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 3:36) | |
| A reaction: This looks to me like one of those regular cases where the formal definitions capture all the logical behaviour of the concept that are required for inference, while failing to fully capture the concept for ordinary conversation. |
| 13206 | A 'linear or total ordering' must be transitive and satisfy trichotomy [Enderton] |
| Full Idea: A 'linear ordering' (or 'total ordering') on A is a binary relation R meeting two conditions: R is transitive (of xRy and yRz, the xRz), and R satisfies trichotomy (either xRy or x=y or yRx). | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 3:62) |
| 13200 | Note that {Φ} =/= Φ, because Φ ∈ {Φ} but Φ ∉ Φ [Enderton] |
| Full Idea: Note that {Φ} =/= Φ, because Φ ∈ {Φ} but Φ ∉ Φ. A man with an empty container is better off than a man with nothing. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 1.03) |
| 13199 | The empty set may look pointless, but many sets can be constructed from it [Enderton] |
| Full Idea: It might be thought at first that the empty set would be a rather useless or even frivolous set to mention, but from the empty set by various set-theoretic operations a surprising array of sets will be constructed. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 1:02) | |
| A reaction: This nicely sums up the ontological commitments of mathematics - that we will accept absolutely anything, as long as we can have some fun with it. Sets are an abstraction from reality, and the empty set is the very idea of that abstraction. |
| 13203 | The singleton is defined using the pairing axiom (as {x,x}) [Enderton] |
| Full Idea: Given any x we have the singleton {x}, which is defined by the pairing axiom to be {x,x}. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 2:19) | |
| A reaction: An interesting contrivance which is obviously aimed at keeping the axioms to a minimum. If you can do it intuitively with a new axiom, or unintuitively with an existing axiom - prefer the latter! |
| 13202 | Fraenkel added Replacement, to give a theory of ordinal numbers [Enderton] |
| Full Idea: It was observed by several people that for a satisfactory theory of ordinal numbers, Zermelo's axioms required strengthening. The Axiom of Replacement was proposed by Fraenkel and others, giving rise to the Zermelo-Fraenkel (ZF) axioms. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 1:15) |
| 13205 | We can only define functions if Choice tells us which items are involved [Enderton] |
| Full Idea: For functions, we know that for any y there exists an appropriate x, but we can't yet form a function H, as we have no way of defining one particular choice of x. Hence we need the axiom of choice. | |
| From: Herbert B. Enderton (Elements of Set Theory [1977], 3:48) |
| 9038 | We must distinguish what the speaker denotes by a name, from what the name denotes [Evans] |
| Full Idea: There are two related but distinguishable questions concerning proper names: what the speaker denotes (upon an occasion), and what the name denotes. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §I) | |
| A reaction: I don't think any account of language makes sense without this sort of distinction, as in my favourite example: the password is 'swordfish'. So how does language gets its own meanings, independent of what speakers intend? |
| 5824 | How can an expression be a name, if names can change their denotation? [Evans] |
| Full Idea: We need an account of what makes an expression into a name for something that will allow names to change their denotations. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §II) | |
| A reaction: Presumably an example would be 'The Prime Minister is in the building'. Evans proposes to discuss communication, rather than strict meanings and descriptions. |
| 9042 | A private intention won't give a name a denotation; the practice needs it to be made public [Evans] |
| Full Idea: Intentions alone don't bring it about that a name gets a denotation; without the intention being manifest there cannot be the common knowledge required for the practice. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §II) | |
| A reaction: Well, I might have a private name for some hated colleague which I mutter to myself whenever I see her. The way names, and language generally, becomes ossified is by joining the great impersonal sea of the language. ..waves of bones, |
| 9041 | The Causal Theory of Names is wrong, since the name 'Madagascar' actually changed denotation [Evans] |
| Full Idea: Change of denotation is decisive against the Causal Theory of Names. Changes of denotation actually occur: a hearsay report misunderstood by Marco Polo transferred the name 'Madagascar' from a portion of the mainland to the African island. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §I) | |
| A reaction: This doesn't sound decisive, as you could give an intermediate causal account of Marco Polo's mistake. I might take the famous name Winston, and baptise my son with it. And I might have done it because I thought Winston was a German dictator. |
| 5825 | Speakers intend to refer to items that are the source of their information [Evans] |
| Full Idea: In general, a speaker intends to refer to the item that is the dominant source of his associated body of information. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §II) | |
| A reaction: This sounds like a theory of reference which fully preserves the spirit of traditional empiricism. Speakers refer to ideas which connect to the source of their underlying impressions. |
| 5823 | The intended referent of a name needs to be the cause of the speaker's information about it [Evans] |
| Full Idea: A necessary (but not sufficient) condition for x's being the intended referent of S's use of a name is that x should be the source of the causal origin of the body of information that S has associated with the name. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §I) | |
| A reaction: This is Evans's adaptation of Kripke's causal theory of names. This cries out for a counterexample. I say something about General Montgomery, having just listened to 'Monty's Double' give a talk, believing it was Montgomery? |
| 9039 | If descriptions are sufficient for reference, then I must accept a false reference if the descriptions fit [Evans] |
| Full Idea: The strong thesis (that descriptions are sufficient for reference) is outrageous. It would mean that if Mr X is wrongly introduced to me as Mr Y, then I truly say 'this is Mr Y' if X overwhelmingly satisfies descriptions of Y. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §I) | |
| A reaction: [I omit some qualifying phrases] Evans says that probably no one ever held this view. It seems right. In the case of an electron it would seem that all the descriptions could be the same, except space-time location. Same electron as yesterday? |
| 9043 | We use expressions 'deferentially', to conform to the use of other people [Evans] |
| Full Idea: Sometimes we use expressions with the overriding intention to conform to the use made of them by some other person or persons. I shall say we use the expression 'deferentially'; examples might be 'viol' or 'minuet'. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §II) | |
| A reaction: I presume Evans wasn't very musical. This label sounds useful, if you wish to connect Grice's account of meaning with Putnam's externalist account of concepts, where deference to experts is crucial. Is all linguistic usage deferential? |
| 9040 | Charity should minimize inexplicable error, rather than maximising true beliefs [Evans] |
| Full Idea: I think the Principle of Charity (maximise true beliefs) is unacceptable. The acceptable principle enjoins minimizing the attribution of inexplicable error and cannot be operated without a theory of the causation of belief for the creatures investigated. | |
| From: Gareth Evans (The Causal Theory of Names [1973], §I) | |
| A reaction: The normal principle of charity certainly seems on shaky ground if you think you have encountered a fairly normal tribe, when they in fact are in possession of the weirdest belief system on the entire planet. |
| 20695 | God's eternal power and deity are clearly seen in what has been created [Paul] |
| Full Idea: From the creation of the world God's invisible nature, namely his eternal power and deity, are clearly perceived in the things that have been made. | |
| From: St Paul (06: Romans [c.55], 19-21), quoted by Brian Davies - Introduction to the Philosophy of Religion | |
| A reaction: St Paul says that for this reason the Gentiles are 'without excuse' for not believing (which means they are in trouble if Christians ever gain political power). Davies says it is unusual to find an argument for God's existence in the Bible. |