Design

Some mathematicians are geniuses and can do entire pages of equations in their heads

Most, however, need to actually write them out

Trying to design in your head without sketching or writing is like trying to solve a new maths problem in your head, just because

That's just self sabotage; our capacity for visualisation and problem-solving is limited, and should be treated like RAM. Since we can't buy additional memory banks (yet lol), so what you should do is put every bit of reasoning on paper

Every line on paper is so many bytes freed up in your mind, and also something objective that can be repurposed and communicated to clients or other designers to let them chip in

I don’t think it’s impossible 
I have friends that are great artists but can’t visualize an art, they can only reproduce what they’ve seen and I have friends that are great at visualizing an idea and finalizing it all in their head, and then putting down the piece 
I think it’s more dependent on the individual