Meditations, Book 7, Section 30

Marcus Aurelius — The Emperor

The most important thing in every person's nature is to care about the common good. The second is to resist bodily desires and impulses. The rational mind has the power to control itself so that neither physical sensations nor appetites can overpower it. Both of these are animal instincts. The mind should rule over them and cannot bear to be controlled by them when it's working properly. This is right, because by nature the mind was meant to command the whole body. The third thing that defines human nature is to avoid being hasty or reckless, and not to fall into error. If the mind focuses on these things and moves forward without getting distracted by other matters, it reaches its purpose and finds happiness.

Marcus Aurelius — The Emperor Original

That therefore which is chief in every man's constitution, is, that he intend the common good. The second is, that he yield not to any lusts and motions of the flesh. For it is the part and privilege of the reasonable and intellective faculty, that she can so bound herself, as that neither the sensitive, nor the appetitive faculties, may not anyways prevail upon her. For both these are brutish. And therefore over both she challengeth mastery, and cannot anyways endure, if in her right temper, to be subject unto either. And this indeed most justly. For by nature she was ordained to command all in the body. The third thing proper to man by his constitution, is, to avoid all rashness and precipitancy; and not to be subject to error. To these things then, let the mind apply herself and go straight on, without any distraction about other things, and she hath her end, and by consequent her happiness.