A surgical instrument comprising: an end effector having a selectively reciprocatable implement movably supported therein, said implement being selectively advanceable in a distal direction upon application of a rotary actuation motion thereto and retractable in a proximal direction upon application of a rotary retraction motion thereto; an elongate shaft assembly comprising: an output shaft coupled to said end effector and configured to transmit said rotary actuation motion and rotary retraction motion to said reciprocatable implement; a firing drive (284, 285) comprising: an external input shaft; and an internal input shaft selectively movable relative to the external input shaft to place a transmission in either a first configuration or a second configuration; and the transmission, which is operable to apply the rotary actuation motion to the output shaft when in the first configuration, and to apply the rotary retraction motion to the output shaft when in the second configuration, said transmission comprising a pinion gear engageable with the output shaft, wherein in said first configuration: said internal input shaft is mated with said output shaft, and and said output shaft is disengaged from the pinion gear such that rotation of said external input shaft is not transmitted to said output shaft via said pinion gear; and wherein in said second configuration: said internal input shaft is disengaged from said output shaft, and said external input shaft and said output shaft are mated via said pinion gear. an end effector having a selectively reciprocatable implement movably supported therein, said implement being selectively advanceable in a distal direction upon application of a rotary actuation motion thereto and retractable in a proximal direction upon application of a rotary retraction motion thereto; an elongate shaft assembly comprising: an output shaft coupled to said end effector and configured to transmit said rotary actuation motion and rotary retraction motion to said reciprocatable implement; a firing drive (284, 285) comprising: an external input shaft; and an internal input shaft selectively movable relative to the external input shaft to place a transmission in either a first configuration or a second configuration; and an output shaft coupled to said end effector and configured to transmit said rotary actuation motion and rotary retraction motion to said reciprocatable implement; a firing drive (284, 285) comprising: an external input shaft; and an internal input shaft selectively movable relative to the external input shaft to place a transmission in either a first configuration or a second configuration; and an external input shaft; and an internal input shaft selectively movable relative to the external input shaft to place a transmission in either a first configuration or a second configuration; and the transmission, which is operable to apply the rotary actuation motion to the output shaft when in the first configuration, and to apply the rotary retraction motion to the output shaft when in the second configuration, said transmission comprising a pinion gear engageable with the output shaft, wherein in said first configuration: said internal input shaft is mated with said output shaft, and and said output shaft is disengaged from the pinion gear such that rotation of said external input shaft is not transmitted to said output shaft via said pinion gear; said internal input shaft is mated with said output shaft, and and said output shaft is disengaged from the pinion gear such that rotation of said external input shaft is not transmitted to said output shaft via said pinion gear; and wherein in said second configuration: said internal input shaft is disengaged from said output shaft, and said external input shaft and said output shaft are mated via said pinion gear. said internal input shaft is disengaged from said output shaft, and said external input shaft and said output shaft are mated via said pinion gear.