when he was 15 what did victor see that made him doubt that the ancient philosophers who had so intrigued him could ever provide real knowledge?

Victor witnessed a terrible thunderstorm and a fire that resulted from lightening striking an oak tree. 

When I was about fifteen years old we had retired to our house near Belrive, when we witnessed a most violent and terrible thunderstorm. It advanced from behind the mountains of Jura, and the thunder burst at once with frightful loudness from various quarters of the heavens. I remained, while the storm lasted, watching its progress with curiosity and delight. As I stood at the door, on a sudden I beheld a stream of fire issue from an old and beautiful oak which stood about twenty yards from our house; and so soon as the dazzling light vanished, the oak had disappeared, and nothing remained but a blasted stump. When we visited it the next morning, we found the tree shattered in a singular manner. It was not splintered by the shock, but entirely reduced to thin ribbons of wood. I never beheld anything so utterly destroyed.