Watch deeds, not words. Seneca, Epistles 5.45.6-8

Seneca urges Lucilius to pay more attention to action than to words, and to be especially careful of accepting flattering appearances (whether of others, or in ourselves: what seems too good to be true is often a vice pretending to be virtue, an enemy wearing the cloak of friendship). The important thing is to act carefully, not to craft verbal arguments (especially not the kind of verbal arguments that make good action harder to find rather than easier).

Quid mihi vocum similitudines distinguis, quibus nemo umquam nisi dum disputat captus est? Res fallunt: illas discerne. Pro bonis mala amplectimur; optamus contra id quod optavimus; pugnant vota nostra cum votis, consilia cum consilis. Adulatio quam similis est amicitiae! Non imitatur tantum illam sed vincit et praeterit; apertis ac propitiis auribus recipitur et in praecordia ima descendit, eo ipso gratiosa quo laedit: doce quemadmodum hanc similitudinem possim dinoscere. Venit ad me pro amico blandus inimicus; vitia nobis sub virtutum nomine obrepunt: temeritas sub titulo fortitudinis latet, moderatio vocatur ignavia, pro cauto timidus accipitur. In his magno periculo erramus: his certas notas imprime. Ceterum qui interrogatur an cornua habeat non est tam stultus ut frontem suam temptet, nec rursus tam ineptus aut hebes ut nesciat tu illi subtilissima collectione persuaseris. Sic ista sine noxa decipiunt quomodo praestigiatorum acetabula et calculi, in quibus me fallacia ipsa delectat. Effice ut quomodo fiat intellegam: perdidi lusum. Idem de istis captionibus dico; quo enim nomine potius sophismata appellem? Nec ignoranti nocent nec scientem iuvant.

Why do you analyze the images drawn by my words, pictures by which nobody is ever taken in save as he examines & disputes them? Real deeds are failing, falling flat around us as we speak: parse them instead. We are embracing evils as if they were good, choosing now against that which we have before chosen. Our vows come to blows with one another, as do our counsels. How similar to friendship is worship! Not content merely to imitate friendship, worship conquers & exceeds it. Into open and eager ears worship is received, and then she descends deep, into our innermost bowels, all the more pleasant to us by the pain her blow brings. Teach me how I am to parse this metaphor properly. Lo! A fawning foe approaches me in the guise of a friend. Vices wearing the name of virtues ambush us. Over there lurks hubris, beneath the sign of strength. Moderation is called cowardice. The fainthearted is taken for a careful man. Here in the midst of these lies we wander, in great danger because of them. Fix clear marks upon them. Meanwhile, ask a man whether he has horns, and he is not so foolish as to feel his own forehead, nor so useless or stupid that he remains ignorant of what you have already managed to persuade him to believe, by the most subtle combination of reasons imaginable. Such games deceive us without harm, like the cups and pebbles of slick street-performers, midst which I find delight in my own deception. Make me understand how this happens, and I lose the fun. I say the same for the obnoxious piffle that you allude to: what else am I going to call such quibbling arguments? They bring no harm to the ignorant, and no aid to the knowing.