Were the disciples unable to drive the boy's demon out because it wasn't a demon but rather an illness (i.e. epilepsy)?
Mark 9:14 - 29
AMP - 14 And when they came to the [nine] disciples, they saw a great crowd around them and scribes questioning and disputing with them. 15 And immediately all the crowd, when they saw Jesus [returning from the holy mount, His face and person yet glistening], they were greatly amazed and ran up to Him [and] greeted Him.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Thomas Cox
All the above answers have been excellent. May I add another possibility that takes more literal approach? There are forms of epilepsy (recurring seizures) that can be controlled by fasting- short term starvation that puts the body into a state of ketosis. The brain requires glucose for energy. It can be forced to use ketones instead of glucose for functioning during this starvation condition. This transformation can cause the cessation of seizing. However, being in ketosis may not cure the underlying problem, thus needing God's healing powers for a total permanent cure. Thus the reason for Christ's statement of this type needing prayer as well as fasting! I just love how God's word can be taken literally and all of the spiritual implications that can be implied. Hope this helps someone and thankful to be able to contribute. BTW, you can look up Ketogenic Diet for more information. It saved my daughters live, along with many peoples prayers!
Danny Hickman
This had nothing to do with a medical condition. Jesus cast a demon out of the boy. Full stop!! Why talk about the brain? The Bible is clear here. The boy was possessed by an unclean spirit. Jesus spoke to the spirit and told it to come out of the boy and to never enter him again. Why talk about medical conditions?
The reason the disciples couldn't cast the demon out is because they tried to do it themselves. That's why Jesus said that kind is only driven out with prayer. He meant their unbelief, their doubt of God's power.
Why would they try to cast the demon out by themselves if they believed God would do it for them? When we pray we are enlisting God's help. When we don't pray we are relying on ourselves. They were relying on themselves. That's what Jesus was saying to them.