Hey!
In my SoP I stated very clearly that I wanted to work on how one topic is analyzed by one author, and everything went out pretty well. However, I am an international student, so maybe this does not apply to Americans: while I chose my area of interest after a five-year degree in Philosophy and an undergrad thesis of 150 pages, some students from the US leave college with just 5 or 6 philosophy seminars! Maybe writing a little bit about the courses that helped you define your area if interest can help.