He tried to read, but could not fix his mind upon the book.
You will have to fix your mind from here to there.
"Could you, only for a moment, fix your mind on something not yourself?"
In these days he could never fix his mind on any one subject for more than a few moments at a time.
He could fix his mind only on one subject and matters related to it.
He fixed his mind on the ultimate goal, made himself stronger.
I tried, as an experiment, to fix my mind on something definite.
"Try, whenever this happens, to fix your mind on something solid and real."
But lastly I fixed my mind on the thought of horses and what those might mean to us.
With his mind fixed on Lexie, he hadn't even noticed.