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?](https://japanese.stackexchange.com/questions/18363/what-are-the-
uses-of-%E3%81%AB-and-%E3%81%A8%E3%81%AF-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?




Yes, when introducing a topic/defining something. You can [find the definition
here](http://dictionary.goo.ne.jp/thsrs/17197/meaning/m0u/).

> 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).




Yes, when introducing a topic/defining something. You can [find the definition
here](http://dictionary.goo.ne.jp/thsrs/17197/meaning/m0u/).

> 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).

