A dictionary of Proverbs (Oxford Paperback Reference)

EXAMPLE is better than precept
Cf. SENECA Letters vi. 5 longum iter est per precepta, breve et efficax per exempla, long is the way by means of precepts, short and effective by examples.
□1400 J. MIRK Festial (EETS) 216 Then saythe Seynt Austeyn [Augustine] that an ensampull yn doyng ys mor commendabull then ys techyng other [or] prechyng. a 1568 R. ASCHAM Schoolmaster (1570) I. 20 One example, is more valiable.. than xx. preceptes written in bookes. 1708 M. PRIOR Literary Works (1971) I. 535 Example draws where Precept fails, And Sermons are less read than Tales. 1828 D. M. MOIR Mansie Wauch xix. Example is better than precept, as James Batter observes. 1981 P. O’DONNELL Xanadu Talisman ii. Example is always better than precept, remember. ■ example, good and bad; words and deeds
The EXCEPTION proves the rule
‘The very fact of an exception proves there must be a rule’ (Brewer); now frequently misunderstood and used to justify inconsistency. Cf. L. exceptio probat regulam in casibus non exceptis, the exception confirms the rule in cases not excepted.

• 
  1640 G. WATTS Bacon’s Advancement of Learning VIII. iii. Exception strengthens the force of a Law in Cases not excepted. 1664 J. WILSON Cheats A2^V I think I have sufficiently justifi’d the Brave man, even by this Reason, That the Exception proves the Rule. 1765 S. JOHNSON Shakespeare Preface C2^V There are a few passages which may pass for imitations, but so few that the exception only confirms the rule. 1863 W. S. GILBERT in Cornhill Mag. Dec. VIII. 727 As for the dictum about Temple Bar, why, the case of Poddle and Shaddery might be one of those very exceptions whose existence is necessary to the proof of every general rule. 1907 H. W. FOWLER Si Mihi 80 It is one of those cryptic sayings, like ‘The exception proves the rule’, which always puzzle me. 1994 ‘C. AIRD’ ‘Fair Cop’ in Injury Time (1995) 27 [H]ome was where most victims of murder met their end. This instance, he was prepared to concede, might just be the exception that proved the rule. ■ rules, general