Truth cannot be known to mortals*

If truth cannot be known to mortals, why is the word in our vocabulary?

*The words of Klytämnestra, in “Electra”.
Opera in one act, by Richard Strauss (1864–1949).
Clytemnestra, in Greek mythology, is a goddess, wife of Agamemnon, and mother of Electra, Chrysothemis, and Orestes