Perhaps my first philosophical though came while I was out on a trail ride in Australia. There 3 or 4 of us, and we were riding on a road made of cracked shells. We passed a house where the grass had just been cut, and someone was cooking meat on a grill.
My horse and I both raised our heads a sniffed the air at the same time. As this was happening, the thought passed through my mind that maybe it was actually the same thing we were smelling. Maybe he interpreted it as grass, and I interpreted it as grilled steak, but it was actually the same smell.
That filled my mind for the whole rest of the ride. What if everything is actually the same, but is interpreted differently by everyone?
It was monumental enough that I remember it like it was yesterday, over 60 years later.