I've seen とは used for the first time and understand from the explanation given on this question, What are the uses of に and とは in this sentence? that とは is (or at least in one specific type of context) is used to mean to/from, and as such makes a kind of comparison.
However, I don't see how that applies to this sentence:
人生とはなにかを考えた。
The most meaningful thing I can imagine this would means is something like: I thought of something relating to life - which is still a bit iffy to me.
Is there another use of とは when there doesn't seem to be comparison?