Seneca, Letters to Lucilius
94.69-71 (tr. Richard M. Gummere):
 It is a great part of health to have forsaken the counsellors of madness and to have fled far from a companionship that is mutually baneful. That you may know the truth of my remark, see how different is each individual's life before the public from that of his inner self. A quiet life does not of itself give lessons in upright conduct; the countryside does not of itself teach plain living; no, but when witnesses and onlookers are removed, faults which ripen in publicity and display sink into the background.  Who puts on the purple robe for the sake of flaunting it in no man's eyes? Who uses gold plate when he dines alone? Who, as he flings himself down beneath the shadow of some rustic tree, displays in solitude the splendour of his luxury? No one makes himself elegant only for his own beholding, or even for the admiration of a few friends or relatives. Rather does he spread out his well-appointed vices in proportion to the size of the admiring crowd.  It is so: claqueurs and witnesses are irritants of all our mad foibles. You can make us cease to crave, if you only make us cease to display. Ambition, luxury, and waywardness need a stage to act upon; you will cure all those ills if you seek retirement.
 Magna pars sanitatis est hortatores insaniae reliquisse et ex isto coitu invicem noxio procul abisse. Hoc ut esse verum scias, aspice quanto aliter unusquisque populo vivat, aliter sibi. Non est per se magistra innocentiae solitudo nec frugalitatem docent rura, sed ubi testis ac spectator abscessit, vitia subsidunt, quorum monstrari et conspici fructus est.  Quis eam quam nulli ostenderet induit purpuram? quis posuit secretam in auro dapem? quis sub alicuius arboris rusticae proiectus umbra luxuriae suae pompam solus explicuit? Nemo oculis suis lautus est, ne paucorum quidem aut familiarium, sed apparatum
vitiorum suorum pro modo turbae spectantis expandit.  Ita est: inritamentum est omnium in quae insanimus admirator et conscius. Ne concupiscamus efficies si ne ostendamus effeceris. Ambitio et luxuria et inpotentia scaenam desiderant: sanabis ista si absconderis.