When considering the three dimensions of space and our freedom of movement in that space, does our ability to change our state of motion (acceleration) imply an existence of a fourth dimension, ie. four space-time dimensions? Given three dimensions each mutually perpendicular, we can move up, down, left, right, forward and backward. We can also rotate around each axes. Is a fourth axes perpendicular to the other three necessary for going faster and slower? I know it's often debated whether time is really a physical dimension or just an abstraction. I also know that with regard to 4-D space-time, time isn't a dimension separate from the other three in an absolute sense; it depends on your orientation within space-time. To summarize my question: is a fourth dimension implicit by the fact that we can accelerate and would it be possible to change the motion of any object without this dimension?