Is it possible to do this?

Assuming a person has a very good knowledge in theory (knows scales, chords, notes on any fret on any string instantly) and is very well ear trained in intervals, would that person be able to compose songs and riffs and solos in their head without ever picking up a guitar and know exactly how it would sound out if they did play it?
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I have no knowledge in theory I dont even know scales, and I can imagine the more frequently played frets on my bass ( the sounds) I can kindoff jam in my head
I've got a friend who plays piano, he's not only the best musician I know, he composes entire pieces for harmony bands (not sure if that's the english word...). He told me that if he hears something in his head (like we all have sometimes) he knows exactly how to play it, so he can compose large parts of pieces without sitting behind his piano.
I believe this is possible for guitar as well
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I'm writing a blog on composing if you wanna check it out, I'll post some of my tips on composing. Yes, its possible. You need to really know guitar and have been composing for a while. I'm able to do it sometimes, but not all the time.


(Not a forum, a blog)
I don't know if it's just me but that's the only way I can write anything decent! When I actually sit down with the guitar & noodle around with scales I get stuck in a rut most of the time. But if I sit in silence & come up with melodies in my head & then work them out on guitar they turn out so much better because you're concentrating on the actual sound, not the physical aspect of playing the sounds if you get me? You should definatley try sitting down in silence, maybe putting a backing track on & coming up with riffs or melodies in your head & then working them out, it's a lot of fun & much easier than you think
it is very possible to do this. It is usually how I begin to write a song. I get an idea of how I want the song to sound, and if you know some things about theory you can make an educated guess as to the effect that certain chords will have on the emotion of the piece. It sounds complicated, but it's really very simple once you figure it out. And it is extremely useful.
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This sometimes happens to me when i'm about to go to sleep - i get my best ideas then... i get this amazing riff or chord sequence, and i'm just imagining playing it on guitar (although sometimes it can be off a bit), i can't remember it for the next day, so i quickly get up and record the idea before i go to sleep... weird