[pre-posting UPDATE: While I was typing, a bunch of people posted some of the stuff I was going to say, but I hope this is helpful.]
The problem is that, if Inform doesn’t think you can reach or touch the Idea to begin with, then the game won’t even understand a command that refers to the idea. So a rule along the lines of “If the player takes an idea…” won’t work, because you don’t even get to the rules for taking an idea – the game won’t even process the option “TAKE IDEA,” because it doesn’t understand “IDEA” as something you can interact with at the moment.
[It also won’t work because you don’t want the rule to start “If the player takes an idea…” If you’ve got a rule for taking an idea, it should begin something like “Before taking an idea…” or “Instead of taking an idea…” or “Carry out taking an idea…” See chapters 7, 12, and 18 of the documentation – hopefully you won’t need to read all the way through all three of those to understand what I’m saying, though!]
Inform has some complicated rules to decide whether something is visible (which basically means you can refer to it in a command) or touchable. That’s why it’s saying you can’t set that directly; Inform wants to be able to calculate that by itself. If you want to take something that wouldn’t be visible otherwise, and make it visible so that the player can refer to it in a command, you have to place it in scope. The way to do this is described in section 17.27 of the manual.
That won’t let you take the idea, though, because in order to take something you have to be able to touch it. Section 12.18 might help here.
I think that if you put these together, it may do what you want. I tried something along these lines and it seemed to work:
Brain is a room. An idea is a thing in Brain.
After deciding the scope of the player when taking or examining: Place the idea in scope.
Rule for reaching inside Brain: allow access.
(“When taking or examining” means you can take the idea, or examine the idea, but you can’t put it on the table. Actually a better thing to do would probably be to write a rule that blocks any action involving the idea other than taking or examining, as discussed here.)
You’ll note that this is pretty complicated! Another thing you could do [pre-posting UPDATE: as Felix says] is actually put the idea in the room, but make it scenery (so the game doesn’t print “You can see an idea here”) and portable (so you can take it); though then you’d only be able to take the idea in that room. You’ll also want to make sure people can’t do anything outlandish with the idea once they have it, unless you want them to. And you might want to think of a way [pre-posting UPDATE: as I4L says] to let people do something with “think” or similar commands, because most people aren’t going to try “take idea.” After all, ordinary ideas can’t be picked up in real life.