"I wonder...now that's the most important question to which I hope to find an answer: can man control his destiny? Can he change the shape of things to come?"
George asks this with his colleagues around at the beginning of the film. He desires to be a man who takes control of his destiny, rather than believing it is up to fate.
"When I speak of time, I'm speaking of the fourth dimension."
George is presenting his scientific findings to his colleagues. He talks about time here, but the importance of it is that he has discovered a way to move within the dimension of time, something that has never been done before.
"If that machine can do what you say it can do, destroy it, George! Destroy it before it destroys you!"
David tells George this as a friend. He knows George will stop at nothing to prove his invention works. In addition, with this David intuits the dangerous game George is playing as he travels into time.
