What answer are we to make to the reflexion that pleasure belongs to good and bad men alike, and that bad men take as much delight in their shame as good men in noble things? This was why the ancients bade us lead the highest, not the most pleasant life, in order that pleasure might not be the guide but the companion of a right-thinking and honourable mind; for it is Nature whom we ought to make our guide: let our reason watch her, and be advised by her. To live happily, then, is the same thing as to live according to Nature: what this may be, I will explain.
If we guard the endowments of the body and the advantages of nature with care and fearlessness, as things soon to depart and given to us only for a day; if we do not fall under their dominion, nor allow ourselves to become the slaves of what is no part of our own being; if we assign to all bodily pleasures and external delights the same position which is held by auxiliaries and light-armed troops in a camp; if we make them our servants, not our masters—then and then only are they of value to our minds.
A man should be unbiased and not to be conquered by external things: he ought to admire himself alone, to feel confidence in his own spirit, and so to order his life as to be ready alike for good or for bad fortune. Let not his confidence be without knowledge, nor his knowledge without steadfastness: let him always abide by what he has once determined, and let there be no erasure in his doctrines.
It will be understood, even though I append it not, that such a man will be tranquil and composed in his demeanour, high-minded and courteous in his actions. Let reason be encouraged by the senses to seek for the truth, and draw its first principles from thence: indeed it has no other base of operations or place from which to start in pursuit of truth: it must fall back upon itself. Even the all-embracing universe and God who is its guide extends himself forth into outward things, and yet altogether returns from all sides back to himself. Let our mind do the same thing: when, following its bodily senses it has by means of them sent itself forth into the things of the outward world, let it remain still their master and its own.
By this means we shall obtain a strength and an ability which are united and allied together, and shall derive from it that reason which never halts between two opinions, nor is dull in forming its perceptions, beliefs, or convictions. Such a mind, when it has ranged itself in order, made its various parts agree together, and, if I may so express myself, harmonized them, has attained to the highest good: for it has nothing evil or hazardous remaining, nothing to shake it or make it stumble: it will do everything under the guidance of its own will, and nothing unexpected will befall it, but whatever may be done by it will turn out well, and that, too, readily and easily, without the doer having recourse to any underhand devices: for slow and hesitating action are the signs of discord and want of settled purpose.
You may, then, boldly declare that the highest good is singleness of mind: for where agreement and unity are, there must the virtues be: it is the vices that are at war one with another.