Proving gratuitous evil

Chapter 5, the largest by far, is devoted to parrying the objection to theism from the existence of natural evil. Rescher has in mind a defense, not a theodicy. He tries to show that, for all the atheist knows, God has, in fact, created the best world possible (so far as the natural order is concerned). It's not that God has created a perfect world (here is invoked the Augustinian notion that created things are by nature finite and imperfect), but that (for all we know) He's created an optimal world. The argument turns on three claims: 1) the world is very, very complex; 2) all of the world's features are so intimately connected that any change has ramifications throughout the system; 3) the world is (in the technical sense) chaotic: a tiny change here can lead to (unpredictable) enormous changes far away in space and time. 

When, then, the atheist suggests that God could have removed some particular evil or type of evil and so made a better world by, for example, changing the laws of nature or initial conditions a bit, Rescher's reply is that it's not enough for the atheist to suggest such possibilities. In order to make her case, the atheist must provide a blueprint for the allegedly better world, complete with a tracing-out of all the long-range consequences of any such proposed alteration in the natural order. And, because of the world's complexity, the atheist can't hope to satisfy that demand. Ergo -- for all anyone knows -- the world may indeed be optimal, the best that a perfect God could have created. 

In any event, Rescher considers another tack that the atheist might try. Why confine oneself to tinkering with our world in search of improvements? Why not let imagination roam freely over much different possible worlds that God would have had better reason to create than this one? Alas for the atheist, Rescher thinks, this maneuver will offer no help - for it won't do merely to partially imagine such a world. One needs to nail down and evaluate the goodness of all the details -- again an impossible task for creatures such as we. One might complain of God that He should have made us smarter, so that we could think such matters through and discover that our world is indeed the best. This complaint might seem ineffectual as an objection to Rescher; he will naturally point out that our intellectual limitations may well be part of what makes this world -- in ways obscure to us -- optimal. 

http://ndpr.nd.edu/news/40454-reason-and-religion/