How to narrow a broad interest into a college essay topic

RReaders in Stanford's admissions office trust an applicant who can hold up one well-chosen piece of a field for inspection, and they slide past one who writes about the field itself. Students who accept that the main essay should be about what they want to study often stall right here. They write about mathematics, or history, or literature, and the essay comes out as a statement of interest a thousand other applicants could have signed. The trouble is altitude: a field is too high up to see anything, and an essay pitched there gives the reader nothing to repeat in committee. Getting down to the right height takes a method, and most students are never shown one.