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The key to it all is that the dual categories, whatever they might be, are absolute. They are separated by a equally absolute boundaries. Thus, by the logic of moral dualism it makes perfect sense to say, "You've either got to be wholly for something or wholly against it." To do otherwise, again by the logic of moral dualism, would be immoral and illogical, it would be a denial of reality. Thus, the more nuanced approach to abortion taken by some, which is both pro-life and pro-choice, is nothing less than dualistic nonsense. "Both...and" simply does not exist in the cognitive landscape of dualism.
The problem with dualism is, of course, that the world we live in and we ourselves are a confusing, inconsistent mixture of things. There is very little in the real world that is black and white. We know this. The sad thing about dualism, thus, is that it forces choices that don't have to be made, creates walls that don't have to be built, and distorts decisions that are better made taking into account the reality of the world we live in. It is a sad fact that all three of the great theist religions—Judaism, Islam, & Christianity—are prone to theological and ideological versions of moral dualism (as are their most vocal critics), which leads said adherents to behave and think in ways that effectively deny the very precepts their faiths teach. There must be a better way, one that for Christians is a more Christ-like way. Amen.