Origin
First attested in 1677; Latin: rēcta (the feminine nominative singular form of rēctus, “right”, “proper”) + ratiō (“reason”, “calculation”), a calque of the Ancient Greek ὀρθός λόγος (orthos logos, “right reason”).
Addendum: Term used in a letter to "Tom" by John Locke, October 20, 1659.
"...We are all centuars, and 'tis the beast that carries us, and everyone's recta ratio is but the traverses of his own steps."