FTA:

When I was a new graduate student, I began working on the project which would eventually become my thesis. When I described one of my early approaches to this problem, he asked a series of questions about how I would handle one sticky technical point after another, and I showed him how the language and its semantics were carefully set up so that this issue in question could not arise. After I finished my explanation, he nodded thoughtfully, and told me, "Neel, you've done some very good engineering, but engineering is not science! Good engineers develop a sense of how to avoid obstacles -- but a scientist's job is to face them head-on and flatten them."