Of means of persuading by speaking there are three species: some consist in the character of the speaker; others in the disposing the hearer a certain way; others in the thing itself which is said, by reason of its proving, or appearing to prove the point.
Now, the causes being four, it is the business of the student of nature to know about them all, and if he refers his problems back to all of them, he will assign the "why" in the way proper to his science-the matter, the form, the mover, that for the sake of which.
Anybody can become angry - that is easy, but to be angry with the right person and to the right degree and at the right time and for the right purpose, and in the right way - that is not within everybody's power and is not easy.
Let us be well persuaded that everyone of us possesses happiness in proportion to his virtue and wisdom, and according as he acts in obedience to their suggestion.