In my latest reading of Richard Bauckham's Jesus and the Eyewitnesses: The Gospels as Eyewitness Testimony, Bauckham talks about the reliability of memory and the issue of whether or not the Gospel of John reflects eyewitness testimony.
An argument that skeptics have made is that, even if the Gospels reflect eyewitness testimony, that would not make them authoritative, for memory is not always reliable. Bauckham acknowledges that memory can be unreliable, for we do not always remember things quite as they occurred, plus we can even "remember" events that did not actually happen. But Bauckham draws on psychological insights and argues that some memories are more reliable than others. For example, people tend to remember fairly accurately (not perfectly, but fairly accurately) events that are unique or unusual, events that are important for them, events in which they are emotionally involved, etc. Bauckham maintains that the Gospels contain those kinds of memories.
On the Gospel of John containing eyewitness testimony, Bauckham points out that John 21:24 identifies the Beloved Disciple as the author who witnessed the things about which he writes: "This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true" (KJV). Unlike many scholars, Bauckham maintains that the Beloved Disciple wrote this verse, rather than that the verse is from a later hand. While many argue that the "we" in v 21 distinguishes the Beloved Disciple from those writing the verse, Bauckham says that the Beloved Disciple could have written that verse as part of a community, and he also notes examples in which ancient writings freely used "we", sometimes interchangeably with "I". Bauckham discusses a phenomenon that he calls the "authoritative we", which he sees in the New Testament and other ancient writings.
Against those who contend that the Gospel of John is inauthentic in its claim to authorship, Bauckham acknowledges that there were pseudepigraphical writers in the ancient world, but he does not believe that the Beloved Disciple is one of them. Why, he asks, would someone writing a pseudepigraphical Gospel attribute it to the Beloved Disciple, who was "obscure, unknown in other Gospel traditions, not in a position to advance his claim to be a significant witness to the events of the Gospel story easily, needing to establish his place in the readers' consciousness artfully and gradually, building the credibility on which he can count only at the very end of the Gospel where he reveals his authorship of the work" (page 409)? Why not pick a more well-known apostle, such as Philip, Andrew, or Thomas?
While Bauckham believes that the Beloved Disciple was a witness in the sense that he actually observed Jesus doing stuff, Bauckham also seems open to the possibility that his status as a witness entailed other things: that he was a witness in God's lawsuit as the deity challenged the claims of the world's gods (Second Isaiah), and that he was a spiritual witness in the sense that he was a believer. Some scholars appeal to these definitions of "witness" to argue that the portrayal of the Beloved Disciple as a witness was literary or spiritual rather than referring to his physically witnessing the acts of Jesus. Bauckham acknowledges that there are literary things going on (i.e., seven witnesses in the first phase of the Gospel), but he does not think that rules out the Beloved Disciple being an actual eyewitness.