A professor is not one who knows, but one who professes to know, and [thus] is constantly in the position of inviting challenge. . . . He professes publicly where everyone is invited to come and challenge, [and] at any time he must be willing and able to defend it openly against all comers. The degree is originally a chivalric device-a gauntlet of defiance to all rivals-and not a safe rampart or dug-out for a scholar to hide behind in safe immunity from any challenge.