Discourses, "What is the Matter on Which a Good Man Should be Employed, and in What We Ought Chiefly to Practise Ourselves" (§1)

“The material for the wise and good man is his own ruling faculty: and the body is the material for the physician and the aliptes (the man who oils persons); the land is the matter for the husbandman. The business of the wise and good man is to use appearances conformably to nature: and as it is the nature of every soul to assent to the truth, to dissent from the false, and to remain in suspense as to that which is uncertain; so it is its nature to be moved towards the desire for the good, and to aversion from the evil; and with respect to that which is neither good nor bad it feels indifferent.”