Very nicely explained, Hurkyl,
This one had been bothering me for some time. I have been guilty of 'jumping the gun' myself, though my stance had steadily been softening. I found this;
Theology - The argument against God's omnipotence
"God is omnipotent, i.e. God can do anything which is logically possible. Making a stone which is so heavy that it cannot be moved is logically possible. Therefore God, being omnipotent, can make a stone so heavy that it cannot be moved. But if God makes a stone so heavy that it cannot be moved, then God cannot move it. But if God cannot move that stone, then there is something God cannot do, and hence God is not omnipotent. Thus if God is omnipotent, then God is not omnipotent. But any property which implies its contradictory is self-contradictory. Thus the very notion of God's (or anyone's) being omnipotent is logically impossible (self-contradictory)."
The argument, as presented just above, is an unholy amalgam of two different arguments, one valid, the other invalid. The valid argument is this (where "G" = "God is omnipotent" and "M" = "God makes an immovable stone"):
[edit by BH]...The notation below didn't copy correctly-best see the link below for accuracy.
G M
M ~G
--------------
G ~G
~G
Although the immediately preceding argument is valid, its second premise is false. The true premise is used in this next argument, but this next argument is invalid:
G M
M ~G
--------------
~G
To derive ~G from the latter pair of premises, one would have to add the further premise, M. But so long as M is false, the conclusion ~G remains underivable. God, thus, remains omnipotent provided that God does nothing, e.g. making an immovable stone, which destroys His/Her omnipotence.
(Question: What if God is omnipotent - as some have argued - of logical necessity and exists necessarily, i.e. in every possible world? The answer, I'm pretty sure, is that, under these conditions, God's making a stone so heavy that God cannot move it is a logical impossibility.)
Taken with consent from;
http://www.sfu.ca/philosophy/swartz/modal_fallacy.htm
Copyright © Norman Swartz 1993, 1999