What does Matthew 12:25 mean?

The Pharisees have stated that Jesus' ability to cast out demons came from Satan and not from God (Matthew 12:22–24). It's not the first time they've made such a claim (Matthew 9:34). Apparently, they either said this in private, or in a way which did not draw much attention. Here, again, they seem to be making the accusation outside of earshot of Jesus. Rather than indicating He heard them, this verse reveals that He knew their thoughts. The fact that Jesus was able to read their minds should have been further evidence to the Pharisees that He was the Messiah.

Jesus begins His counterargument against their charge with logic: Kingdoms divided against themselves are defeated. Civil war doesn't make for powerful nations. Divided cities and houses, likewise, do not stand. They fall. The point, one Jesus clarifies in the following verses, is that Satan would not cast out the very demons he sent out to oppress people. He would only be defeating himself.