Matthew 12:26 - Ellicott's Commentary On The Whole Bible

If Satan cast out Satan. — In the Greek the name has the article in both places, as pointing to the one great adversary. It is not that one Satan casts out another, but that he, on the assumption of the Pharisees, casts out himself. Satan is not personally identified with the demon, the deaf or dumb spirit, that had possessed the man, but the language implies that where evil enters into the soul, Satan enters also. (Comp. John 13:27.) There is, as it were, a seeming ubiquity, a solidarité, in the power of evil, as there is admittedly in the sovereign power of good.