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An inconsistent triad consists of three propositions of which at most two can be true. For example:
- Alice loves me.
- If Alice loves me, she would have sent flowers.
- Alice hasn't sent flowers.
If one finds oneself believing all three propositions of an inconsistent triad, then (to be rational) one must give up or modify at least one of those beliefs. Maybe Alice doesn't love me, or maybe she wouldn't send flowers to me if she did, or maybe she actually has sent flowers.
The problem of evil is often given in the form of an inconsistent triad. For example, J. L. Mackie gave the following three propositions:
- God is omnipotent
- God is omnibenevolent
- Evil exists
Mackie argued that these propositions were inconsistent, and thus, that at least one of these propositions must be false. Either:
- God is omnipotent and omnibenevolent, and evil does not exist.
- God is omnipotent, but not omnibenevolent; thus, evil exists by God's will.
- God is omnibenevolent, but not omnipotent; thus, evil exists, but it is not within God's power to stop it (at least not instantaneously).
Many responses have been made to the problem of evil, including the proposition that evil exists as a consequence of a greater good, such as free will; that evil is an illusion; and that evil is necessary for spiritual growth.
Howard-Snyder, F., Howard-Snyder, D., & Wasserman, R. (2009). The Power of Logic (4th Edition). New York: McGraw-Hill. (p. 336) ISBN 978-0-07-340737-1