Were the disciples "seeing things?"
When we discussed the appearances of Jesus, we talked about how these weren't just vague appearances to a few individuals, but that these were experiences that were to groups. They involved touching, hearing and seeing and were not just brief, confusing episodes. (See Matthew 28:9-10, Luke 24:13-36, John 20:11-18, John 20:24-29, John 21, Acts 1:3, 1 Corinthians 15:3-8)
I might hallucinate due to a lack of sleep, grief or through abuse of drugs but I can't hallucinate the same thing with someone. Hallucinations exist in the individual's mind. They're not group experiences.
These appearances came not to just friends, but foes, including Paul and James. It's hard to argue these non-believers were in the frame of mind to hallucinate the risen Jesus.
Hallucinations also don't explain the evidence of the empty tomb.
Process of Elimination
"Although at least a few if not all of Jesus’ disciples may have been in an emotional state that rendered them candidates for a hallucination, the nature of some of the experiences of the risen Jesus, specifically those that occurred in group settings and to Jesus’ enemy Paul, and the empty tomb strongly suggest that these experiences were not hallucinations.”
- Will Durant, an American writer, philosopher, and historian. Best known for his 11-volume "The Story of Civilization".