fucking pagans!

Schopenhauer and Plato both address this issue in different ways. Plato describes Akrasia as weakness of will, where one knows what reason dictates but fails to pursue that goal. Though he does identify it as a phenomenon he can't explain it. Mostly because, as Schopenhauer points out, it is based on an empirically flawed moral psychology. Reason for Schopenhauer does not motivate us to act, desire is an exclusive function of the Will, which always motivates us to act. Reason is simply an instrument to guide us in efficiently and effectively fulfilling desire.

That doesn't mean rational approaches to ethics have no place. They are best utilized in collective tools like government to assure good outcomes when using public or finite resources.

The great articulator of this debate is Richard Taylor, the beekeeping philosopher, in his book Good and Evil. The most underrated philosophy publication of the last 100 years imo.