Chastity is a treasure which, according to St. Paul, we keep in fragile vessels; and in truth it has much of the fragility of those vases which break by knocking against each other." The freshest water, when we try to preserve it in a vessel, quickly loses its freshness if any animal touches it.
Never permit yourself, Philothea, to practise, and preserve yourself from, those external liberties, equally contrary to Christian modesty and to the respect you owe yourself; for, although one may preserve an absolutely chaste heart in spite of actions which arise rather from want of thought than from malice, and which are not usually practised, nevertheless, chastity always receives from them some lamentable injury. You sufficiently understand that I do not speak here of what virtually ruins chastity.
— St. Francis de Sales, Introduction to the Devout Life, Chapter XIII, how to preserve chastity