seþan

Old English

sēþan

to prove
- late 9th century, King Alfred's translation of Boethius' The Consolation of Philosophy
  Iċ eom ġeþafa þæs þe þū sæġst, for þon þe þū hit hæfst ġesēðed mid ġesċādwīslīcre race.
  I agree with what you're saying, since you have proven it with rational argument.
- c. 992, Ælfric, "The Nativity of St. Paul the Apostle"
  Saulus sōðlīċe miċelum swīðrode and þā Iūdēiscan ġesċende, mid ānrǣdnesse sēðende þæt Crist is Godes sunu.
  Saul only became stronger and embarrassed the Jews, proving with his resilience that Christ is the son of God.
to attest, affirm
- c. 992, Ælfric, "The Nativity of St. James the Apostle"
  Eall hālgu ġewritu sēðaþ þæt sē is Hǣlend Crist.
  All the books of the Bible attest that he is Jesus Christ.

Conjugation of sēþan (weak class 1)

Joseph Bosworth and T. Northcote Toller (1898) “SĒÐAN”, in An Anglo-Saxon Dictionary‎^[1], 2nd edition, Oxford: Oxford University Press.