He conducted a thought experiment to solve a problem in physics.

Do you understand the necessity of this project?

That information is unnecessary.

That information is completely unnecessary.

He thinks that war is a necessary evil.

No problem, I will do that job.

That proposal is completely unthinkable.

As a practical question, how are we going to fund this project?

This exam is a matter of life or death for me.

He is viewing that situation as a serious problem.