This paper will discuss logician Alvin Plantinga's approach to the logical problem of evil. My position is that Plantinga successfully rebuts the charges of this well-known challenge to theism (belief in God).
What the Logical Problem of Evil Tries to Accomplish
The logical problem of evil is a puzzle for both the theistic and non-theistic mind--a conundrum that has its roots in times of antiquity. The "problem" can be formulated thus:
(1) God is omnipotent.
(2) God is omnibenevolent.
(3) Evil exists.
The atheologian J.L. Mackie argues that 1-3 constitute an inconsistent set of (formally) contradictory propositions. Worded simply, Mackie contends that God cannot be omnipotent (almighty) or omnibenevolent (wholly good) if evil obtains. He insists that it is logically problematic for God and evil to exist or obtain simultaneously: Mackie believes that it is not a case of both-and, but either-or. Either God exists or evil exists and ne'er the twain shall meet:
In its simplest form the problem is this: God is omnipotent; God is wholly good; and yet evil exists. There seems to be some contradiction between these three propositions, so that if any two of them were true the third would be false. But at the same time all three are essential parts of most theological positions: the theologian, it seems, at once must adhere and cannot consistently adhere to all three. (The problem does not arise only for theists, but I shall discuss it in the form in which it presents itself for ordinary theism.)
Quote is from J.L. Mackie, "Evil and Omnipotence," Mind, New Series, 64.254. (April 1955): 200-212.
Plantinga's Reply to the Logical Problem of Evil
Conversely, Alvin Plantinga, taking a cue from Augustine of Hippo insists that it is logically possible for God to possess the objective properties of omnipotence and omnibenevolence although evil obtains. He systematically appeals to human free will and modal logic to rebut the atheistic charges of Mackie. Modal logic deals with possibility and necessity: it distinguishes between possible and necessary truths. In summary, Plantinga maintains that 1-3 above are not formally inconsistent propositions when we account for the appropriate modal distinctions. For example, he states that the following proposition is not necessarily true: "A good thing always eliminates evil as far as it can."
The modal term "necessarily" is important in this argument since the next proposition also does not appear to be necessarily true: "There are no limits to what an omnipotent being can do." Yet, in order for Mackie's contention to hold, Plantinga maintains that the aforesaid propositions must be not only true, but necessarily true (i.e., they cannot not be true). These claims must be true in every possible world if Mackie's argument is going to be sustained.
While S (a certain person or subject) might be a good doctor, it does not mean that S always eliminates evil (E) as far as S can. A proposition of this sort does not appear to be necessarily true since there might be substantive reasons why S (a good doctor, in this case) would not eliminate evil insofar as it's possible for S to eliminate E. For example, a doctor who has the power to remove cataracts might not immediately perform surgery on a needful patient until a later time. But this reticence would not mean that the medical professional lacks the ability or the desire to remove the cataracts.
Moreover, it seems that an all-powerful (omnipotent) being might have certain limits, believe it or not. Can an omnipotent entity bring about a contradictory state of affairs such that 2 + 2 = 5 and square circles as well as married bachelors begin to obtain? Not if the law of logical necessity holds. Nor does it seem that a maximally powerful being can change the past without violating the law of noncontradiction. So, while there might be some possible world in which an omnipotent being eliminates suffering or evil altogether, it's possible that there are other worlds in which an omnipotent being does not remove evil, even though such a being would have the requisite power to eradicate evil.
Logical Outcome of Plantinga's Argument in the Light of Scripture
There are evidently good reasons why God has permitted evil. He made rational creatures upright, but we choose our respective paths (Ecclesiastes 7:29). God evidently allows creation to exist in a state of futility until a Messianic figure should arrive to liberate creation from bondage to slavery and corruption (Romans 8:19-21). That is a plausible account of God's permission of evil based on the New Testament and logic. In any event, there are apparently good reasons why God has not eliminated evil at this time (2 Peter 3:9). Plantinga makes a similar case based on his use of modal logic.
God apparently vouchsafed free will to human beings. So does there exist a possible world (counterfactual situation) in which creatures always freely do what is right and never violate the beneficent laws of deity? In the future (based on my reading of Scripture), humans will freely do what is right for all eternity. However, at that time, there will be no devil, no Satanic world nor any sin (Revelation 20:1-10; 21:1-5).
The bottom line: 1-3 do not constitute an inconsistent propositional set. It is logically possible that God would permit evil to obtain for reasons primarily known to God. But we have been given some plausible reasons why evil persists from the pages of Judeo-Christian Scripture.
Peterson, Michael L. Philosophy of Religion: Selected Readings. New York: Oxford University Press, 2001.
Plantinga, Alvin. God, Freedom, and Evil. Grand Rapids: Eerdmans, 1977.