Open-minded scepticism
Campbell's own phrase, and the only stance the model asks you to adopt.
The instruction is symmetric. Without open-mindedness you will never learn anything new, because you already believe you know everything. Without scepticism you will believe anything at all. Either failure on its own is fatal.
What it looks like in use: treat a new idea as a possibility, give it a probability based on your own experience, keep reassessing, gather supporting data without filtering, then go looking for facts that contradict you. Accept only what you can verify for yourself — and be doubly sceptical of your own assessments and conclusions.
This is the reason this app never asks you to believe any of it. My Big TOE, in its own words, is nothing to be believed in; if it is not your experience, it cannot be your truth.
Where this gets misread
That scepticism means rejecting things. It means declining to file anything as settled — including your own conclusions, and including the parts of the model you find most convincing.