The metaphor of the bending moral arc symbolizes what may be the most important and least appreciated trend in human history—moral progress—and its primary cause is one of the most understated sources: scientific rationalism.
By progress I accept the Oxford English Dictionary’s historical usage as “advancement to a further or higher stage; growth; development, usually to a better state or condition; improvement.” By moral I mean “manner, character, proper behavior” (as from the Latin moralitas), in terms of intentions and actions that are right or wrong with regard to another moral agent. Morality involves how we think and act toward other moral agents in terms of whether our thoughts and actions are right or wrong with regard to their survival and flourishing. By survival I mean the instinct to live, and by flourishing I mean having adequate sustenance, safety, shelter, bonding and social relations for physical and mental health. Any organism subject to natural selection—which includes all organisms on this planet and most likely on any other planet as well—will by necessity have this drive to survive and flourish, for if they didn’t they would not live long enough to reproduce and would therefore no longer be subject to natural selection.