Matthew 17:19

Then came the disciples to Jesus apart
Or "secretly", as the Vulgate Latin, and Munster's Hebrew Gospel read; that is, privately, and when alone; and as Mark says, "when he was come into the house"; and was by himself, then came the nine disciples to him, to converse with him about this matter,

and said unto him, why could not we cast him out?
That is, the devil, and so cure the lunatic; the Syriac and Persic versions render it, "why could not we heal him?" The lunatic; which only could be done by casting out the demon: they were concerned, fearing they had lost the power which Christ had bestowed on them, and wanted to know what they had done, which had deprived them of it; and what should be the cause of their late unsuccessful attempt, when they had so frequently triumphed over the unclean spirits, that were subject to them. Though they might have learned from the answer Christ gave to the father of the lunatic, and the general character of the Jewish nations in that answer, the true reason of their own inability; but this they took no notice of, imagining it belonged entirely to others, and not to them.