“He who knows not how to lie, knows not how to live.”
qui nescit dissimulare, nescit vivere.
I read this in my copy of Robert Burton’s The Anatomy of Melancholy, edited by Holbrook Jackson, who in the endnotes attributes this quotation to Frederick Barbarossa. I have also seen a variant of this saying, qui nescit dissimulare, nescit regnare attributed to Louis XI. If anyone knows anything about this maxim, please post it in the comments!