The library's note
The puzzle is old. Leibniz was struck that mind and body run in perfect step — the soul "acting according to the laws of final causes," the body "according to the laws of efficient causes," yet the two "in harmony with one another" — and he could explain it only by a harmony God had established between them in advance. The modern argument strips away Leibniz's particular metaphysics and turns the harmony itself into evidence.
Consider pain. It feels a certain awful way, and it also makes you withdraw, guard the injury, and remember the danger. Notice these are two different facts: the felt quality, and the behaviour. There seems no necessity that the state driving avoidance should feel bad rather than good — a creature might have been built so that pleasure made it flee and agony drew it in, or so that its inner life had no bearing on its actions at all. Across the space of possible mind-body laws, harmony like ours is a vanishingly rare exception. Yet we have it, top to bottom: our experiences match our behaviour, our senses match the world, our reports match our experiences.
On theism this is expected: a good God, making conscious creatures, would pair their inner lives with their bodies well — pain with harm, pleasure with flourishing, perception with truth. On a mindless naturalism there is no such steering; the psychophysical laws are simply brute, and brute laws had every chance of being discordant. Something enormously improbable on one view and expected on the other has, its defenders argue, turned out exactly as the God-view predicts.
Two objections carry the weight. The first is functionalism: if a mental state simply IS its functional role — if to be pain just is to be the state that does what pain does — then harmony is automatic, not a lucky match, and there is no gap to explain. The second, pressed by Graham Oppy and others, is that even if harmony needs explaining, a good God is only one of several "value-oriented" explanations, so the argument may reach a benevolent ground of things without reaching the full God of theism. How much survives these is exactly what the current debate — all of it recent, and under copyright — is about.
Written for this library — not a quoted source. It reports how the premise is argued and answered; the philosophers' own words are above.