Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
The same is true of verbs: for instance fero disappears in the perfect and subsequent tenses. Nor does it matter greatly whether such forms are nonexistent or too harsh to use. For what is the genitive singular of progenies or the genitive plural of spes? Or how will quire and ruere form a perfect passive or passive participles.
Why should I mention other words when it is even doubtful whether the genitive of senatus is senati or senatus? In view of what I have said, it seems to me that the remark, that it is one thing to speak Latin and another to speak grammar, was far from unhappy. So much for analogy, of which I have said more than enough.
Etymology inquires into the origin of words, and
For instance Marcus Caelius wishes to prove that he is homo frugi, not because he is abstemious (for he could not even pretend to be that), but because he is useful to many, that is fructuosus, from which frugalitas is derived. Consequently we find room for etymology when we are concerned with definitions.
Sometimes again this science attempts to distinguish between correct forms and barbarisms, as for instance when we are discussing whether we should call Sicily Triquetra or Triquedra, or say meridies or medidies, not to mention other words which depend on current usage.