Apart ( κατ ἰδι ìαν). Jesus had retired to a house (Mark) when the disciples came to him. The question which they desired to ask was one that could not he investigated in the presence of the sneering, unbelieving crowd.
Why could not we ( ἡμεῖς, emphatic) cast him ( αὐρο Ì, it) out? They had keenly felt their impotence and failure, so publicly and distressingly displayed, especially as they had received power to eject demons, and had successfully exercised this authority (Luke 10:17).
The Lord's rebuke (Matthew 17:17) had passed over their heads, and not been understood as applicable to themselves. So it was with some bitterness that they asked the question. The nine had not been permitted to witness the Transfiguration; they were not even to be made acquainted with this wondrous transaction at present.
More preparation, greater receptivity, was required, before they were fit to be admitted to the full mysteries of the kingdom. They had still much to learn, were still only pupils, and their late failure was permitted in order to help them to attain to self-knowledge and more entire self-surrender.