19 ideas
| 13030 | Extensionality: ∀x ∀y (∀z (z ∈ x ↔ z ∈ y) → x = y) [Kunen] |
| Full Idea: Axiom of Extensionality: ∀x ∀y (∀z (z ∈ x ↔ z ∈ y) → x = y). That is, a set is determined by its members. If every z in one set is also in the other set, then the two sets are the same. | |
| From: Kenneth Kunen (Set Theory [1980], §1.5) |
| 13032 | Pairing: ∀x ∀y ∃z (x ∈ z ∧ y ∈ z) [Kunen] |
| Full Idea: Axiom of Pairing: ∀x ∀y ∃z (x ∈ z ∧ y ∈ z). Any pair of entities must form a set. | |
| From: Kenneth Kunen (Set Theory [1980], §1.6) | |
| A reaction: Repeated applications of this can build the hierarchy of sets. |
| 13033 | Union: ∀F ∃A ∀Y ∀x (x ∈ Y ∧ Y ∈ F → x ∈ A) [Kunen] |
| Full Idea: Axiom of Union: ∀F ∃A ∀Y ∀x (x ∈ Y ∧ Y ∈ F → x ∈ A). That is, the union of a set (all the members of the members of the set) must also be a set. | |
| From: Kenneth Kunen (Set Theory [1980], §1.6) |
| 13037 | Infinity: ∃x (0 ∈ x ∧ ∀y ∈ x (S(y) ∈ x) [Kunen] |
| Full Idea: Axiom of Infinity: ∃x (0 ∈ x ∧ ∀y ∈ x (S(y) ∈ x). That is, there is a set which contains zero and all of its successors, hence all the natural numbers. The principal of induction rests on this axiom. | |
| From: Kenneth Kunen (Set Theory [1980], §1.7) |
| 13038 | Power Set: ∀x ∃y ∀z(z ⊂ x → z ∈ y) [Kunen] |
| Full Idea: Power Set Axiom: ∀x ∃y ∀z(z ⊂ x → z ∈ y). That is, there is a set y which contains all of the subsets of a given set. Hence we define P(x) = {z : z ⊂ x}. | |
| From: Kenneth Kunen (Set Theory [1980], §1.10) |
| 13034 | Replacement: ∀x∈A ∃!y φ(x,y) → ∃Y ∀X∈A ∃y∈Y φ(x,y) [Kunen] |
| Full Idea: Axiom of Replacement Scheme: ∀x ∈ A ∃!y φ(x,y) → ∃Y ∀X ∈ A ∃y ∈ Y φ(x,y). That is, any function from a set A will produce another set Y. | |
| From: Kenneth Kunen (Set Theory [1980], §1.6) |
| 13039 | Foundation:∀x(∃y(y∈x) → ∃y(y∈x ∧ ¬∃z(z∈x ∧ z∈y))) [Kunen] |
| Full Idea: Axiom of Foundation: ∀x (∃y(y ∈ x) → ∃y(y ∈ x ∧ ¬∃z(z ∈ x ∧ z ∈ y))). Aka the 'Axiom of Regularity'. Combined with Choice, it means there are no downward infinite chains. | |
| From: Kenneth Kunen (Set Theory [1980], §3.4) |
| 13036 | Choice: ∀A ∃R (R well-orders A) [Kunen] |
| Full Idea: Axiom of Choice: ∀A ∃R (R well-orders A). That is, for every set, there must exist another set which imposes a well-ordering on it. There are many equivalent versions. It is not needed in elementary parts of set theory. | |
| From: Kenneth Kunen (Set Theory [1980], §1.6) |
| 13029 | Set Existence: ∃x (x = x) [Kunen] |
| Full Idea: Axiom of Set Existence: ∃x (x = x). This says our universe is non-void. Under most developments of formal logic, this is derivable from the logical axioms and thus redundant, but we do so for emphasis. | |
| From: Kenneth Kunen (Set Theory [1980], §1.5) |
| 13031 | Comprehension: ∃y ∀x (x ∈ y ↔ x ∈ z ∧ φ) [Kunen] |
| Full Idea: Comprehension Scheme: for each formula φ without y free, the universal closure of this is an axiom: ∃y ∀x (x ∈ y ↔ x ∈ z ∧ φ). That is, there must be a set y if it can be defined by the formula φ. | |
| From: Kenneth Kunen (Set Theory [1980], §1.5) | |
| A reaction: Unrestricted comprehension leads to Russell's paradox, so restricting it in some way (e.g. by the Axiom of Specification) is essential. |
| 13040 | Constructibility: V = L (all sets are constructible) [Kunen] |
| Full Idea: Axiom of Constructability: this is the statement V = L (i.e. ∀x ∃α(x ∈ L(α)). That is, the universe of well-founded von Neumann sets is the same as the universe of sets which are actually constructible. A possible axiom. | |
| From: Kenneth Kunen (Set Theory [1980], §6.3) |
| 19419 | Not all of perception is accompanied by consciousness [Leibniz] |
| Full Idea: I do not think that the Cartesians have ever proved or can prove that every perception is accompanied by consciousness. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.195) | |
| A reaction: This idea is very important in Leibniz, because non-conscious or barely conscious thoughts and perceptions explain a huge amount about behaviour, reality and morality. |
| 19421 | Souls act as if there were no bodies, and bodies act as if there were no souls [Leibniz] |
| Full Idea: Everything takes place in souls as if there were no body, and everything takes place in bodies as if there were no souls. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.198) | |
| A reaction: I don't think I have ever encountered a modern thinker who accepts this view. Leibniz rejected Occasionalism, but his account depends entirely on the role of God, to set up the pre-established harmony. Why would God do that? |
| 468 | Musical performance can reveal a range of virtues [Damon of Ath.] |
| Full Idea: In singing and playing the lyre, a boy will be likely to reveal not only courage and moderation, but also justice. | |
| From: Damon (fragments/reports [c.460 BCE], B4), quoted by (who?) - where? |
| 19420 | Death and generation are just transformations of an animal, augmented or diminished [Leibniz] |
| Full Idea: Death, like generation, is only the transformation of the same animal, which is sometimes augmented and sometimes diminished. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.195) | |
| A reaction: Leibniz has a very unusual view of death, since neither minds nor their bodies can ever be wholly destroyed. Death is a kind of shrinking. I suspect that he was wrong about that. |
| 19416 | Not all of matter is animated, any more than a pond full of living fish is animated [Leibniz] |
| Full Idea: It must not be said that each portion of matter is animated, just as we do not say that a pond full of fishes is an animated body, although a fish is. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.190) | |
| A reaction: This is a particularly clear picture of the role of monads in matter. Monads are attached to bodies, which are entirely inanimate, but monads suffuse matter and give it its properties, like particularly bubbly champagne. Cf Idea 19422. |
| 19422 | Every particle of matter contains organic bodies [Leibniz] |
| Full Idea: There is no particle of matter which does not contain organic bodies. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.198) | |
| A reaction: Cf Idea 19416. There seems to be an interaction problem here (solved, presumably, by pre-established harmony). The organic bodies are there to explain the active behaviour of matter, but the related matter seems intrinsically inert. |
| 19418 | Mechanics shows that all motion originates in other motion, so there is a Prime Mover [Leibniz] |
| Full Idea: The maxim that there is no motion which has not its origin in another motion, according to the laws of mechanics, leads us again to the Prime Mover. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.194) | |
| A reaction: This is Leibniz's endorsement (uncredited) to Aquinas's First Way. It is hard to see how the laws of mechanics could have anything to say about the origin of movement. And doesn't the law say that the motions of God need a mover? |
| 19417 | All substances are in harmony, even though separate, so they must have one divine cause [Leibniz] |
| Full Idea: My system of Pre-established Harmony furnishes a new proof of God's existence, since it is manifest that the agreement of so many substances, of which the one has no influence upon the other, could only come from a general cause on which they all depend. | |
| From: Gottfried Leibniz (Principle of Life and Plastic Natures [1705], p.192) | |
| A reaction: Adjacent harmony seems self-explanatory, but remote harmony is interesting evidence for God. Hence modern quantum non-locality should make us all wonder whether there is a deeper explanation. |