This is a really interesting idea because it attempts to adequately explain the behaviour of the disciples of Jesus following his death. The suggestion is that each of the disciples were overcome with grief and disappointment when Jesus was brutally murdered, that their minds were affected in some way such that they began to hallucinate and start seeing Jesus alive again – or so they thought. We know that the disciples genuinely believed that they had seen Jesus alive after his death, and this would account for that. So how does this theory fare?
Despite it looking like a promising explanation, there are some fairly significant problems with this view.
First of all, it is very uncommon – unprecedented perhaps – for more than one person to hallucinate about exactly the same thing; far less a small community of people. Secondly, these appearances were not simple individual experiences. Groups of people, at least on one occasion numbering 500, experienced mass, mutual hallucinations of Jesus at the same time. This is, by all accounts, unheard of. By definition, hallucinations are from within the mind, being internal phenomenon when a personal imagination perceives something that doesn’t actually exist. These appearances, shared by more than one person at the same time, are therefore not easily explained by appealing to hallucinations.
Thirdly, hallucinations are a product of a person’s own thoughts. You don’t hallucinate about something of which you have no concept. Think about it like this – young children only make up role-play based on experiences that they have had, (shops, making dinner, going to the doctors, etc), or on things they may have seen or read about, (trains, boats, astronauts, etc), but don’t suddenly play pretend tax accountants or enact a microbe, for example. The time will come when they learn about tax, accountancy and microbiology, but until then, they have no space in their minds for such concepts and in the same way, your mind can only work with the concepts that are within it. Now, for the disciples, they did not have any expectation or concept of the immediate resurrection of a crucified Messiah.
It stretches credulity therefore to believe that the minds of hundreds of people each produced visions of Jesus, sometimes simultaneously, despite them not having a pre-existing concept of the resurrection of the Messiah.
Finally, and perhaps crucially, it is one thing to believe that the followers of Jesus had visions of him in their imaginations, but it is another thing entirely to think that James and Paul also suffered from mental instability of some kind enough for them to hallucinate and believe they had seen Jesus alive again. The odds are indeed highly stacked against hallucinations being a worthy explanation for the events of nearly 2,000 years ago.
 1 Corinthians 15:6
 Merriam-Webster Medical Definition: “a perception of something (as a visual image or a sound) with no external cause”