Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
He must be ready to answer questions and to put them unasked to those who sit silent. In praising the recitations of his pupils he must be neither grudging nor over-generous: the former quality will give them a distaste for work, while the latter will produce a complacent self-satisfaction.
In correcting faults he must avoid sarcasm and above all abuse: for teachers whose rebukes seem to imply positive dislike discourage industry.