grammar - Use of とは when there doesn't seem to be comparison

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?

Answer

Yes, when introducing a topic/defining something. You can find the definition here.

定義{ていぎ}・[主題化]{しゅだいか}を表わす (To indicate a definition or topic)

So based on that, a literal translation of 人生とはなにかを考えた。 would be something like "I thought about the definition of life".

You can find this usage a lot, so I highly recommend remembering it. Even a quick google of とは you will find tons of hits for various sites. You can also use it as a type of question marker like T-SQLとは? meaning "What is T-SQL?". I find it's usage in wikipedia or dictionary sites often as they give definitions for things.

Also, note that とは is a rigid form usually used in writing and the more colloquial form is というのは.

There also is another usage of とは when used to expressed that you are surprised. For example, こんなところにあったとは！ to mean "I can't believe it was in this place!" (for example in astonishment when you are going to a place you never are been before and surprised at where it is located). * Note this version cannot be replaced by というのは like the first definition).