Some comments from Grenville Kleiser’s "The Training of a Public Speaker"

The orator should consider what the subject is upon which he is to speak, before whom, for whom, against whom, at what time, in what place, under what conditions, what the public think of it, what the judges may think of it before they hear him, and what he himself has to desire, and what to apprehend. Whoever makes these reflections will know where he should naturally begin. But now orators call exordium anything with which they begin, and consider it of advantage to make the beginning with some brilliant thought. Undoubtedly many things are taken into the exordium which are drawn from other parts of the cause or at least are common to them, but nothing in either respect is better said than that which can not be said so well elsewhere.