the word "challenge"

[Casiopea]

It was something he wanted to prove or justify. It was a challenge; something he wanted to test out.

Which do you would answer, "doubt" or "test", if asked the word "challenge"
below?
Please answer me!!



　Richard Feynman, who was awarded the Nobel Prize for Physics in 1965, says about his attitude toward what he had read as a university student:
　　
　　I was inspired by the remarks in those books; not by the parts in which everything was proved and demonstrated but by the remarks about the fact that this doesn’t make any sense. So I had this as a (i)challenge and an inspiration.