The sum of his doctrine concerning philosophy, in general, is this: Philosophy is the exercise of reason in the pursuit and attainment of a happy life; whence it follows, that those studies which conduce neither to the acquisition nor the enjoyment of happiness are to be dismissed as of no value. The end of all speculation ought to be, to enable men to judge with certainty what is to be chosen, and what to be avoided, to preserve themselves free from pain, and to secure health of body, and tranquillity of mind. True philosophy is so useful to every man, that the young should apply to it without delay, and the old should never be weary of the pursuit; for no man is either too young or too old to correct and improve his mind, and to study the art of happiness. Happy are they who possess by nature a free and vigorous intellect, and who are born in a country where they can prosecute their inquiries without restraint: for it is philosophy alone which raises a man above vain fears and base passions, and gives him the perfect command of himself.