The Complete Works of Claudius Galen, Vol. 18 Pt. 2

through these commentaries to go through only those things which are useful, and to examine and scrutinize again at greater leisure in another treatise those matters which were argued by Herophilus against the Prognostic of Hippocrates. And he who narrates the things which the sick omit is believed to understand the conditions of those who are ill all the more. It behooves those who attempt to explain anything of such matters to dare to write thus only after having first demonstrated it in practice. For it is easy for anyone to write what he wishes, but it is very difficult to set forth such a theory from which one will succeed, when speaking of the sick, in identifying not only the things that are about to happen, but also the things that have already happened—at which he himself was not present—and the things that are now occurring concerning the sick, which he neither heard from them nor knows himself. Just as we, in the things we shall say, might deservedly be considered credible, having previously demonstrated the truth of the theory by the facts themselves. But so that I may not be forced to write about the same things twice, both in the present explanation of the prologue and...

[Latin translation:]

...by refuting them to delay. And so I think I shall act most conveniently if I pursue only useful matters in these commentaries, but later in another work, when there is more leisure, I examine and explore what Herophilus wrote against the Prognostic of Hippocrates. And he who explains what they omit is believed to understand the condition of the sick most correctly. Those who attempt to explain anything of this sort ought not to apply their mind to writing until they have demonstrated the thing itself by their work. For it is easy for anyone to write what you wish, but it is very difficult to explain a theory and method of such a kind that he who not only predicts the future for the sick, but also reports the past, at which he was not present while it was occurring, and similarly those things which are now happening to the patient, which he neither learned from him nor saw himself, might be for the most part truthful. But credence cannot be rashly given to our writings, as we have already proven by the thing itself how true this theory is. However, lest the things which have been written by us in the explanation of this preface, later when...