Verse 40. - And I besought thy disciples to cast him out; and they could not. This appears to have been a case of the deadliest kind of epileptic lunacy. Our Lord distinctly assumes here that the disease in this case was occasioned by an unclean spirit who had taken possession of the suffering child. The whole question of demoniacal possession, its extent, its cause, whether or no it still survives in some of the many mysterious phases of madness, is very difficult. It has been discussed elsewhere (see notes on Luke 4:33 and following verses).

9:37-42 How deplorable the case of this child! He was under the power of an evil spirit. Disease of that nature are more frightful than such as arise merely from natural causes. What mischief Satan does where he gets possession! But happy those that have access to Christ! He can do that for us which his disciples cannot. A word from Christ healed the child; and when our children recover from sickness, it is comfortable to receive them as healed by the hand of Christ.And I besought thy disciples,.... The nine disciples that were left behind, whilst Christ, and the other three, were gone up to the mountain:

to cast him out; the devil out of his child:

and they could not; See Gill on Matthew 17:16.