Were the disciples "seeing things?"
Hallucinations are false perceptions of things that are not there that are private and subjective.
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, conversation and seeing and were not just brief, confusing episodes. Luke reports that these appearances lasted for a period of 40 days. These visions for some were repeated experiences. (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)
Hallucinations may be caused by a lack of sleep, physical illness, 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. The disciples themselves were also skeptical at first, demonstrating they were not likely to suffer a hallucination. (Luke 24:11, 24:25, Matthew 28:17)
Hallucinations don't explain the evidence of the empty tomb. As Peter Kreeft points out: "The apostles could not have believed in the "hallucination" if Jesus' corpse had still been in the tomb. This is a very simple and telling point; for if it was a hallucination, where was the corpse? They would have checked for it; if it was there, they could not have believed."
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".