The old-school way, with the chief scout having it all in his head, gave no continuity. If he gets run over by a bus he takes all the knowledge with him.
"Kids disappear into imaginary relationships all the time" . . . but "usually there's a limit to how far that imaginary relationship can go, because it's all in their head."
When a neurologist ruled out medical causes like Lyme disease, Ms. Abaspour recalled, her husband said, “I think we should just give her a placebo — it’s all in her head.”
In the senses of "within one's mind" and "within one's imagination", sometimes preceded by all as an intensifier emphasizing that the indicated information, belief, etc. is entirely and exclusively within one's mind or imagination.