“It’s an exciting development. I did not know that they were coming out with not one but two of these [quantum computing results] in the same week,” says Scott Aaronson, a theoretical computer scientist at the University of Texas at Austin. “That's pretty insane.”

Source: Scientific American China Is Pulling Ahead in Global Quantum Race, New Studies Suggest

Does "That's pretty insane" mean "That's pretty amazing"? Or does it mean "That's pretty foolish"? The former is informal while the latter is formal (normal sense of "insane"). I am not sure.

  • 6
    To be clear, "that's pretty insane" by itself can either be meant in a positive way (amazing) or a negative way (foolish). It's the additional context of the paragraph that should leads you to conclude the former, not something inherent in the phrase.
    – BradC
    Jul 23 at 13:22
  • 1
    French (délirant, probably fou) and German (irre, verrückt) have similar uses. Jul 24 at 13:51
  • No one can explain it better than Cypress Hill: youtube.com/watch?v=RijB8wnJCN0 Jul 24 at 18:48

The sense is surprising, in a positive way. That can be seen in the first sentence:

It’s an exciting development.

Merriam-Webster insane

4 informal : also : exceptionally good or impressive
Minnesota … has blazed out to an insane start, taking an early lead in the Northwest Division.

  • 7
    +1, but I think that Merriam's definition could be improved by adding the additional qualifier of exceptionally good or impressive to a degree that makes it almost unbelievable or incredible. The implication being that, if you didn't know better, you might have to almost be insane to believe that such an amazing thing just happened.
    – J...
    Jul 23 at 16:03
  • I think the emphasis is not that it is surprising so much as it is extraordinary to the degree of unbelievable (which would be a stand-in). Of course everything extraordinary is by implication surprising, but that's not the focus. Jul 24 at 13:56

It's closest in meaning to "pretty amazing" :

4. exceptionally good or impressive (Merriam-Webster, 4)

The other definitions don't fit because they all have negative meanings, and in this context, where China is moving ahead of the US in an important field, it doesn't make sense to call China or anybody in the story "crazy".

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