Or I should say in this forum: From whom one bounces ideas. Weaker than a co-author or collaborator. Not a confidant (no element of privacy or secrecy). Friend is too broad. Suggestions?
2I'm incredibly tempted to call them a rubber duck, although I'm fairly certain that's limited exclusively to a programming context.– SomethingDarkMay 8, 2017 at 1:01
1If this was in a professional setting, you might call them consultants.– LawrenceMay 8, 2017 at 1:05
I would say sounding board.
In an artistic context, muse might fit the bill:
Source: http://www.oed.com/view/Entry/124057 (2.c.)
Maybe, although to me, an artist’s “muse” is a woman who inspires someone else with her beauty. The mythical Muses supposedly gave Homer and Hesiod their ideas. Neither meaning would convey to me someone whose role is to listen to someone else’s ideas and give feedback.– DavislorSep 22, 2021 at 21:29
I would say a "Foil". Usually used to refer to characters in a story that compliment a more main character but I believe it works here too. I also think "foiling" or being a good foil for ideas is a practical and useful skill for an "Ideonomist", or someone who studies Ideas.
Although I would probably call this person a “sounding-board” as well, or say I “bounced ideas off” someone, writers sometimes call them a “beta reader.”