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.