Taxonomy.
The classification of biological forms is a very involved, ecletic field. It requires a very detailed and precise study of physical form, lifestyle, genetics, and even some behavioral observations. Once a detailed conceptual picture is taken of an organism, it is then necessary to fit it in with other very detailed conceptual pictures of other organisms. Even the final process of classification involves an elegant process of making distinctions fine enough to account for any and all differences between a certain species and another. This is especially striking when it comes to microorganisms and insects.
It was a one-off I took for one semester way-back-when - but it had a deep effect on me: if you want to be accurate in any field, in any respect it requires detailed knowledge and an ability to make relevant, precise distinctions.