Here is a model:Domain: {1, 2, 3, 4, ...}Referents: a : 64 b : 2 c : 111Extension of S : {1, 4, 9, 16, 25, ...}Extension of P : ∅Extension of D : {h1, 2i ,h 2, 4i ,h 3, 6i ,h 4, 8i ,h 5, 10i ...}(So for every number 1, 2, 3, ... in the domain, the extension of D contains an ordered pair with that number in first position and the number twice its size in second position.)State whether the following propositions are true or false on the model, and briefly explain your answers.(i) ∀x((Sx ∧ ∃wDwx) → x = a)(ii) ∃z(z 6 = b ∧ P z) ↔ ∃x∀y(Dyx ∨ Dxc)
Question
Here is a model:Domain: {1, 2, 3, 4, ...}Referents: a : 64 b : 2 c : 111Extension of S : {1, 4, 9, 16, 25, ...}Extension of P : ∅Extension of D : {h1, 2i ,h 2, 4i ,h 3, 6i ,h 4, 8i ,h 5, 10i ...}(So for every number 1, 2, 3, ... in the domain, the extension of D contains an ordered pair with that number in first position and the number twice its size in second position.)State whether the following propositions are true or false on the model, and briefly explain your answers.(i) ∀x((Sx ∧ ∃wDwx) → x = a)(ii) ∃z(z 6 = b ∧ P z) ↔ ∃x∀y(Dyx ∨ Dxc)
Solution
(i) False. The proposition ∀x((Sx ∧ ∃wDwx) → x = a) can be translated as "For all x, if x is in S and there exists a w such that D contains the ordered pair (w,x), then x equals a". In the given model, a equals 64. However, there are other numbers in S (like 1, 4, 9, 16, 25, ...) that also have corresponding pairs in D (like (1,2), (2,4), (3,6), ...). Therefore, it's not true that every x that is in S and has a corresponding pair in D must equal 64 (a).
(ii) False. The proposition ∃z(z 6 = b ∧ P z) ↔ ∃x∀y(Dyx ∨ Dxc) can be translated as "There exists a z that is not equal to b and is in P if and only if there exists an x for all y such that D contains the ordered pair (y,x) or D contains the ordered pair (x,c)". In the given model, b equals 2 and P is empty. Therefore, there cannot exist a z that is not equal to 2 and is in P, because there are no elements in P. On the other hand, there can exist an x for all y such that D contains the ordered pair (y,x) or D contains the ordered pair (x,c), because D contains ordered pairs for every number in the domain. Therefore, the two sides of the biconditional are not equivalent, making the proposition false.
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