Judge the value, not the reality
Once anything starts happening — an image, a knowing, a sense of someone else present — the mind produces one question immediately: did I just make that up, or was that real? Campbell's response is unusually direct. Wrong thing to ask. Do not ask that.
His reasoning is that all of it is information, and information is as real as it gets. It arrives from the larger system, from other consciousnesses, or from you — and there is no flag on any of it saying which. All three can give you something valuable and all three can give you rubbish. So you judge the value, not the reality. The useful question is not is it real. It is is it useful.
There is also a practical reason not to ask mid-experience: doing so ends the experience. The moment you wonder whether you imagined it, your intellect is back in charge, the being level is gone, and you are dead in the water. Judging and analysis go afterwards. Always.
Afterwards means considerably later than you would like. His standard is not one instance but ten or twenty, over a month or two, and then you ask whether it has been useful — keeping what has, discarding what has not. He identifies the common failure precisely: people want to decide how real something was before they have enough data to decide anything.
And the quiet mind turns out to be load-bearing for exactly this. Your mind must be calm, clear and steady, he writes, or you cannot tell the difference between what your mind creates and what exists independently of it. That is why it was the first step.
Campbell, Fireside Chat, October 2018; My Big TOE, Book 1(opens in a new tab)