- #1
darkchild
- 155
- 0
This proof makes no sense to me.
The theorem to be proved is
Theorem 44. {x,y} = {u,v} → (x = u & y = v) V (x = v & y = u)
where {x,y} and {u,v} are sets with exactly two members, which can be either sets or individuals. The proof relies on:
Theorem 43. z [itex]\in[/itex] {x,y} z = x V z = y.
The given proof is:
"By virtue of Theorem 43
(1)
(2)
Case 1: x = y. Then by virtue of (1), x = u, and by virtue of (2), y = v."
This is where I got lost. Couldn't I just as easily argue that x = v by virtue of (2) and y = u by virtue of (1)? Or by virtue of (3) and (4)? What's the rationale behind the assumed values of x and y, and couldn't any of the four propositions support it? And if x = y, how could I justify the argument that x and y were equal to two ostensibly different variables without showing that u = v? On one, hand I can sort of see that the assumption x = y and the conditions (1) - (4) would necessarily make it true that u = v...but, given that then either x or y could be said to be equal to either v or u, would there be any need for this part of the proof at all?
In the interest of completeness, the rest of the proof is:
Case 2: x ≠ y. In view of (1), either x = u or y = u. Suppose x ≠ u. Then y = u and by (3) x = v. On the other hand, suppose y ≠ u. Then x = u and by (4), y = v.
The theorem to be proved is
Theorem 44. {x,y} = {u,v} → (x = u & y = v) V (x = v & y = u)
where {x,y} and {u,v} are sets with exactly two members, which can be either sets or individuals. The proof relies on:
Theorem 43. z [itex]\in[/itex] {x,y} z = x V z = y.
The given proof is:
"By virtue of Theorem 43
u [itex]\in[/itex] {u,v},
and thus by the hypothesis of the theorem u [itex]\in[/itex] {x,y}.
Hence, by virtue of Th. 43 again (1)
u = x V u = y.
By exactly similar arguments(2)
v = x V v = y,
(3) x = u V x = v,
(4) y = u V y = v.
We may now consider two cases.Case 1: x = y. Then by virtue of (1), x = u, and by virtue of (2), y = v."
This is where I got lost. Couldn't I just as easily argue that x = v by virtue of (2) and y = u by virtue of (1)? Or by virtue of (3) and (4)? What's the rationale behind the assumed values of x and y, and couldn't any of the four propositions support it? And if x = y, how could I justify the argument that x and y were equal to two ostensibly different variables without showing that u = v? On one, hand I can sort of see that the assumption x = y and the conditions (1) - (4) would necessarily make it true that u = v...but, given that then either x or y could be said to be equal to either v or u, would there be any need for this part of the proof at all?
In the interest of completeness, the rest of the proof is:
Case 2: x ≠ y. In view of (1), either x = u or y = u. Suppose x ≠ u. Then y = u and by (3) x = v. On the other hand, suppose y ≠ u. Then x = u and by (4), y = v.