Discourses, "On the Power of Speaking" (§3)

“But if you ask me what then is the most excellent of all things, what must I say? I cannot say the power of speaking, but the power of the will, when it is right. For it is this which uses the other (the power of speaking), and all the other faculties both small and great. For when this faculty of the will is set right, a man who is not good becomes good: but when it fails, a man becomes bad. It is through this that we are unfortunate, that we are fortunate, that we blame one another, are pleased with one another. In a word, it is this which if we neglect it makes unhappiness, and if we carefully look after it, makes happiness.”