Charles Spurgeon Commentary Mark 9:17-18

"And one of the multitude answered him, Teacher, I brought unto thee my son, who hath a dumb spirit; and wheresoever it taketh him, it dasheth him down: and he foameth, and grindeth his teeth, and pineth away: and I spake to thy disciples that they should cast it out; and they were not able." — Mark 9:17-18 (ASV)

And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit; and wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away:

A case of dreadful epilepsy accompanied with satanic possession.

And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit; and wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not.

It was an aggravated case of epilepsy, attended with possession by an evil spirit. The disciples could not cast out this devil, and the scribes had therefore attacked their faith in the Master himself while he was away.