A phrase, word, or idiom to describe the person who reviews as an outsider
I am looking for a phrase, word, or idiom to describe my current status in one scenario, which gave me the advantage in spotting a mistake and correcting it. I will give some example other than what we had in real:
- I carry out some mathematical calculations spanning into two pages.
- I end up with an impossible result, so I go back and check my steps.
- I fail to see my own mistake, so I call for help.
- Friend who came for help spots the mistake in a matter of seconds, which is immediately obvious to me as soon as it is pointed out.
Assume that my friend is not any more competent in mathematics than I am. The reason why my friend was able to spot the mistake I've made was, however counter-intuitive it is, that he was fresh in dealing with this problem and my mind was uncontrollably taking the shortcuts it had established in re-thinking this exact same problem.
I would use the phrase "foreign eye/perspective" to describe the quality my friend bears which allowed him/her to see what I couldn't. Unfortunately, no such phrase seems to be available in English.
If no such phrase or idiom is available, then the most concise way to put this would also be very appreciated.
A common idiom for this situation is a fresh pair of eyes.