“This is the effect of philosophy, which is the medicine of our souls: it banishes all groundless apprehensions, frees us from desires, and drives away fears: but it has not the same influence over all people; it is of very great influence when it falls in with a disposition well adapted to it. …
For how few philosophers will you meet with, whose life and manners are conformable to the dictates of reason! who look on their profession, not as a means of displaying their learning, but as a rule for their own practice! who follow their own precepts, and comply with their own decrees. …
For just as if one who professed to teach grammar should speak with impropriety, or a master of music sing out of tune, such conduct has the worst appearance in these people, because they blunder in the very particular with which they profess that they are well acquainted. So philosophers who err in the conduct of their life are the more infamous because they are erring in the very thing which they pretend to teach, and, while they lay down rules to regulate life by, they are irregular in their own life.”
(Tusculan Disputations, 2.4)
Cicero on the hypocrisy of certain philosophers