Paul denies that Jesus appeared non-physically (e.g. intra-mentally), insisting instead that what appeared to him was quite physical and extra-mental; it was Jesus in the flesh, and so externally perceptible. It was not a mere vision or epiphany.
This page analyzes these 8 arguments:
This is relevant, because if Paul hallucinated, it would’ve been either through a visual hallucination or non-visual hallucination (e.g. an epiphany), and neither of these is compatible with Paul’s understanding of the event.
If Paul only had a vision (hallucination?) on the road to Damascus, it is improbable that it would have resulted in Paul's converting.
In the purported appearance of Jesus to Paul recorded in Acts, did Paul in fact only see a “light” which he identified as Jesus (rather than seeing Jesus himself)?
This page analyzes this 1 evidence:
This is relevant because a light as such is non-physical.
The Greek “optasia” philologically denotes a denotes a non-physical experience. (See full page here.) This is relevant because Acts 26:19 reports Paul saying “I did not prove obedient to the heavenly vision [optasia]” and the “heavenly vision” was Jesus himself (or the nature of his appearance).
So what? Plausibly…
Paul actually was proclaiming that Jesus appeared inside of him in some sense.
This full page analyzes these 4 arguments:
But so what?,
In writing 2 Corinthians 4:1-6 (“[Jesus] has shone in our hearts”) Paul is he referring to Jesus’ appearing to him?