Kowton & Parris in deep concentration... In accordance with relativity, they are in some sense interchangeable, but we do know that they form co-equal parts of a larger 'thing' called space-time, and it is only within space-time that the most complete understanding of the motion and properties of natural objects and phenomena can be rigorously understood by physicists. Space and time are to space-time what arms and legs are to humans. In some sense they are interchangeable, but you cannot understand 10,000 years of human history without including both arms and legs as part of the basic human condition.