He has also furnished us with a double criterion of truth. One, on the perception of those effects, which are according to imagination; the other, the intelligence of those things which are ethical, and which concern politics, and economy, and laws. The chief good he has defined to be the exercise of virtue in a perfect life. He used also to say, that happiness was a thing made up of three kinds of goods. First of all, the goods of the soul, which he also calls the principal goods in respect of their power; secondly, the goods of the body, such as health, strength, beauty, and things of that sort; thirdly, external goods, such as wealth, nobility of birth, glory, and things like those. And he taught that virtue was not sufficient of itself to confer happiness; for that it had need besides of the goods of the body, and of the external goods, for that a wise man would be miserable if he were surrounded by distress, and poverty, and circumstances of that kind. But, on the other hand, he said, that vice was sufficient of itself to cause unhappiness, even if the goods of the body and the external goods were present in the greatest possible degree. He also asserted that the virtues did not reciprocally follow one another, for that it was possible for a prudent, and just, and impartial man, to be incontinent and intemperate; and he said, that the wise man was not destitute of passions, but endowed with moderate passions. He also used to define friendship as an equality of mutual benevolence. And he divided it into the friendship of kindred, and of love, and of those connected by ties of hospitality. And he said, that love was divided into sensual and philosophical love. And that the wise man would feel the influence of love, and would occupy himself in affairs of state, and would marry a wife, and would live with a king. And as there were three kinds of life, the speculative, the practical, and the voluptuous, he preferred the speculative. He also considered the acquisition of general knowledge serviceable to the acquisition of virtue. As a natural philosopher, he was the most ingenious man that ever lived in tracing effects back to their causes, so that he could explain the principles of the most trifling circumstances; on which account he wrote a great many books of commentaries on physical questions.