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?

In the book Frankenstein at age 15 Victor witnesses a summer thunderstorm that arouses his thoughts about electricity and possible applications for its use. The storm indirectly gives Victor the opportunity to learn more about technology and science. The storm Shelley describes is much like the one she and her fellow writers experience during the summer of 1816. This made him doubt ancient philosophers.