True and rightly ordered love prefers the greater to the lesser good. Now it is clear, that, of all human good, the welfare of the soul is the greatest: next in degree comes physical well-being; and external goods occupy the last place. It is natural to man to observe this order in his preference. For who would not rather lose bodily eyesight than the use of reason? Who would not part with all his property in order to save his life? “Skin for skin,” said Satan to the Lord, “and all that a man has he will give for his life” (Job ii. 4). Very few, if any, fail to observe this order in their preference concerning the natural goods of which we have given examples. There are, nevertheless, many who pervert this order of charity, in the case of the other goods which exist in addition to the purely natural ones of which we have spoken. They will, for instance, prefer physical health or comfort, to the acquisition of virtue or learning; and they will expose their bodies to danger and hardship, in order to gain material wealth. Now this, as we shall show more at large, is not true love. Neither do they who act thus, love themselves sincerely. It is quite clear that the chief part of a thing is really the thing itself. When we say that a city acted thus or thus, we mean that the chief citizens acted in such or such a manner. Now we know, that the principal thing in man is the soul, and that the chief among the powers or faculties of the soul, is the reason or understanding. He, therefore, who despises the good of the rational soul, for the sake of physical welfare, or of the advantage of the sensitive soul, plainly show that he does not truly love himself. “He who loves iniquity, hates his own soul” (Pa. x. 6).
De Perfectione, ch. 13. This directly ties into what it means to love one's neighbor as one's self, of course.