Cicero, De Finibus 1.64
“Unless, moreover, the nature of the world will be grasped completely, there is no way we can justify the judgments of our senses. For, whatever we perceive in our mind arises in each way from our senses. If these sensations are true—as Epicurus’ doctrine instructs—only then may something be known and perceived. Those who undermine this and claim that nothing can be perceived are not able to support their own propositions because they have had the senses themselves removed.
In addition, once knowledge is set aside with science, every kind of reason for leading life and acting is removed. Thus, from natural philosophy, we get strength against the fear of death, constancy against the terror of religion; and peacefulness of mind—once ignorance of the mysteries of nature is removed. We also get self-control when the character and shapes of desire are explained to us. And, as I just demonstrated, by using the canon of knowledge—also set out by Epicurus—we learn to distinguished fact from fiction.”
Nisi autem rerum natura perspecta erit, nullo modo poterimus sensuum iudicia defendere. Quidquid porro animo cernimus, id omne oritur a sensibus; qui si omnes veri erunt, ut Epicuri ratio docet, tum denique poterit aliquid cognosci et percipi. Quos qui tollunt et nihil posse percipi dicunt, ii remotis sensibus ne id ipsum quidem expedire possunt quod disserunt. Praeterea sublata cognitione et scientia tollitur omnis ratio et vitae degendae et rerum gerendarum. Sic e physicis et fortitudo sumitur contra mortis timorem et constantia contra metum religionis et sedatio animi, omnium rerum occultarum ignoratione sublata, et moderatio, natura cupiditatum generibusque earum explicatis, et, ut modo docui, cognitionis regula et iudicio ab eodem illo constituto veri a falso distinctio traditur.