Montesquieu

The jud­ges who had no other rules than prac­ti­ces usually infor­med them­sel­ves about them by wit­nes­ses on each ques­tion that came up.

As judi­cial com­bat was fal­ling into disuse, inqui­ries were conduc­ted in wri­ting. But oral evi­dence writ­ten down is never more than oral evi­dence ; it was only increa­sing the costs of the pro­ce­dure. Statutes were made that ren­de­red most of these inqui­ries need­less1 ; public regis­ters were esta­bli­shed in which most of the facts were cer­ti­fied : nobi­lity, age, legi­ti­macy, mar­riage. Writing is a wit­ness which it is dif­fi­cult to cor­rupt ; they had the cus­toms set down in wri­ting. All that was quite rea­so­na­ble : it is sim­pler to go look up in bap­tis­mal regis­ters whe­ther Pierre is the son of Paul than to go prove that fact with a long inquiry. When in any coun­try there is a very large num­ber of prac­ti­ces, it is sim­pler to write them all down in a code than to oblige indi­vi­duals to prove every prac­tice. Finally, they made the famous ordi­nance that pro­hi­bi­ted the recei­ving of evi­dence by wit­nes­ses for a debt of more than a hun­dred livres, unless there was a begin­ning of proof in wri­ting.

See how age and family were proven in Establishments, book I, ch. lxxi–lxxii.