And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit; A dumb spirit - That is, a demon who afflicted those in whom it dwelt with an incapacity of speaking. The spirit itself could not be either deaf or dumb. These are accidents that belong only to organized animate bodies. See this case explained, Matthew 17:14 (note), etc.
Related Commentaries of Mark 9:17
Mark 9:17
17 And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit;