Albert Barnes Commentary Matthew 17:15

"Lord, have mercy on my son: for he is epileptic, and suffereth grievously; for oft-times he falleth into the fire, and off-times into the water." — Matthew 17:15 (ASV)

Lord, have mercy. The word "Lord" here means Sir, a title of civility, not implying Divinity.

My son. This was an only son (according to Luke). He was possessed with a devil.

This calamity was attended with the following symptoms: he was lunatic (see Barnes on Matthew 4:24); he was sore vexed (that is, he suffered greatly or was greatly afflicted); and he often fell suddenly, in the manner of persons having epileptic fits.

He was also dumb—that is, he was dumb except when the fit was coming on him; for Luke says that when the spirit took him, he cried out suddenly. He foamed at the mouth, gnashed with his teeth, and wasted away, or became poor and emaciated.

Furthermore (according to Luke), the spirit tore him and scarcely departed from him, meaning he had only short intervals of reason. This is because the passage in Luke, bruising him, hardly departeth from him, should be so translated.