You could write about how philosophy has opened your mind to a wide variety of new ideas. Some data that might help, Philosophy majors score higher on entrance tests into university level than the other humanities (and most majors). Greek philosophy is the basis of most western thought. Critical thinking is a constantly evolving skill. You could go into depth with it. Describe what new methods of understanding you have acquired.
By illustrating what you have learned in the instructor's class, you will show a deep understanding to the core of what was being taught (assuming that you learned what the instructor wanted you to learn).