Gold and dross, summer noon and winter night

Meaning

III.13 amplifies the superiority claimed for true philosophy through sharp comparisons: gold and dross, summer noon and winter night, great wealth and extreme poverty. Croesus and Irus give the wealth contrast named human figures. These are scales of value and illumination, not claims that knowledge literally turns an inferior metal into gold or changes the season.

The comparison exalts philosophy’s lovers above other people: Croesus surpasses Irus in wealth, summer noon surpasses winter night in luminous heat, and bright pure gold surpasses the base dross of iron in lustre, value and power. Crasse is the dross, not pure iron, and vertu here is effective power. The source is making an extravagant comparative valuation, not measuring medical efficacy or promising literal seasonal change.