A few years after reading his father’s copy of Kant’s Prolegomena to any Future Metaphysics, the physicist Ernst Mach had this experience which he later wrote about.
“On a bright summer day in the open air, the world with my ego suddenly appeared to me as one coherent mass of sensations,” Mach wrote. He felt, in that moment, there was no reality sitting “out there,” independent of his sensations, and likewise that there was no self sitting “in here,” independent of its sensations. He grew certain that there could be no real difference between mind and matter, between perceiving subject and perceived object. “This moment was decisive for my whole view,” he wrote (from “When Einstein Tilted at Windmills: The young physicist’s quest to prove the theories of Ernst Mach” by Amanda Gefter (nautil.us)).