Thus, whether the gate has always been open or had at one point of time been closed is virtually determined by two elements which complete each other: (1) the existence or extinction of mujtahids, and (2) the jurist’s consensus that the gate of ijtihad, for the reason of extinction, was, or was not closed. In usul works, only the question whether or not mujtahids can, by reason or by shar’, become extinct, was discussed and there had hardly been a direct reference to the concept of ‘the gate of ijtihad’. This is perhaps due to the fact that the usulists, being the guardians of law, felt responsible for the continuity of ijtihad and saw in the whole idea of the gate a negation of the very raison d’être of the divine methodology of usul-al-fiqh.
“Was the Gate of Ijtihad closed?” by Wael B. Hallaq in: International Journal of Middle East Studies, Vol. 16, No. 1 (Mar., 1984), pp. 3-41, at p. 21