(philosophy, logic) In artificial intelligence, modus ponens.

(idiomatic) To seem logical, rational, or reasonable; to make sense.

(US, idiomatic) For reasons unknown and not worth speculating on.

(idiomatic, chiefly in the negative) Logic; common sense.

(philosophy) The use of reason to decide how to act.

(law) A degree of uncertainty to which a reason can be assigned; in those jurisdictions influenced by English common law, the minimum degree of uncertainty required to acquit a defendant.

(Roman Catholicism) Seven years of age, at which age a person is morally liable for the sins that he or she commits.

A doctrine in antitrust law which applies to contracts which "unreasonably" inhibit trade.

(obsolete) To be right. / To have grounds, justification etc. (to do something, or for something).

(nonce word) A twofold or double reason.