"Felix, qui potuit rerum cognoscere causas" is verse 490 of Book 2 of the "Georgics" (29 BC), by the Latin poet Virgil (70 - 19 BC). It is literally translated as: "Fortunate, who was able to know the causes of things". Dryden rendered it: "Happy the Man, who, studying Nature's Laws, / Thro' known Effects can trace the secret Cause" (The works of Virgil, 1697).[1][2]
Virgil may have had in mind the Roman philosopher Lucretius, of the Epicurean school.