Quintilian, Inst. Orat. 3.6.
“I admit that I now have a bit of a different opinion from what I believed before. Perhaps it would be safest for my reputation to change nothing which I not only believed but also approved for many years. But I cannot endure knowing that I misrepresent myself, especially in this work which I compose as some help for our good students. For even Hippocrates, famous still for his skill in medicine, seems to have conducted himself very honorably when he admitted his own errors so his followers would not make a mistake. Marcus Tullius did not hesitate to condemn some of his own books in subsequent publications, the Catulus and Lucullus, for example.
Prolonged effort in research would certainly be useless if we were not allowed to improve upon previous opinions. Nevertheless, nothing of what I taught then was useless. These things I offer now, in fact, return us to basic principles. Thus it will cause no one grief to have learned from me. I am trying only to collect and lay out the same ideas in a slightly more sensible fashion. I want it made known to all, moreover, that I am showing this to others no later than I have convinced myself.”
Ipse me paulum in alia quam prius habuerim opinione nunc esse confiteor. Et fortasse tutissimum erat famae modo studenti nihil ex eo mutare quod multis annis non sensissem modo verum etiam adprobassem. Sed non sustineo esse conscius mihi dissimulati, in eo praesertim opere quod ad bonorum iuvenum aliquam utilitatem componimus, in ulla parte iudicii mei. Nam et Hippocrates clarus arte medicinae videtur honestissime fecisse quod quosdam errores suos, ne posteri errarent, confessus est, et M. Tullius non dubitavit aliquos iam editos libros aliis postea scriptis ipse damnare, sicut Catulum atque Lucullum et… Etenim supervacuus foret in studiis longior labor si nihil liceret melius invenire praeteritis. Neque tamen quicquam ex iis quae tum praecepi supervacuum fuit; ad easdem enim particulas haec quoque quae nunc praecipiam revertentur. Ita neminem didicisse paeniteat: colligere tantum eadem ac disponere paulo significantius conor. Omnibus autem satis factum volo non me hoc serius demonstrare aliis quam mihi ipse persuaserim.