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

...it lacks something to be most certain, and even then it is named in the same way. For this reason, those who explain matters concerning it sometimes speak with an addition, at one time calling it accurate knowledge, at another certain, and at another firm or even continuous, indicating by that very addition that the concept of "certainty" is not always implied by the term "knowledge" itself. Thus, therefore, prognosis is twofold: it is certain that after winter spring will come, then summer, then autumn; but it is not certain in the matters which Aratus wrote concerning the moon:

If its horns rise well,
You must expect the North Wind, but when they are upturned, the South.

For it has been observed that while it turns out this way for the most part, it happens sometimes that it does not. The followers of Herophilus believe that prognosis possesses certainty, but prediction does not. For they say that many of the things predicted do not happen, as if someone could predict something without foreknowing...

...at times it is lacking that which would make it most certain, and not even then does it lose the name. Therefore, those who expound upon it sometimes designate it, with the addition of a qualifier, as accurate, certain, firm, or even perpetual knowledge; indicating by that very addition that a certain state of affairs is not always designated by the term "knowledge." Therefore, foreknowledge is twofold: certain, as that spring will follow winter, then summer, later autumn; but not certain in those things which Aratus wrote concerning the moon:

If the high horns project well,
Expect the force of the North Wind; if they are upturned, they will stir the South.

For so it has been observed to happen for the most part, but yet it happens otherwise at times. However, Herophilus considers foreknowledge to be certain, but prediction not so. For he says that many things are predicted which do not come to pass. As if, indeed, one could predict those things which one does not beforehand...