Philosophic thought is now temporarily influenced by the revival of an old principle known as the principle of relativity. The popular name for this is the Einstein theory, because in 1905 Albert Einstein, working on some theories developed by Lorentz and Fitzgerald, published his first principle of relativity which suggested that the velocity of light is constant, however the position of an observer may vary and that space and time are variable. In 1917 Einstein enlarged this idea in order to include all the laws of nature. Space and time are treated as just mental concepts. They lack the concreteness of matter, but they compose the framing of the universe and give it form and continuity. Consequently we see so much of them that we attribute reality to them. The theory of relativity suggests that time is not continuous. There is no identity of instants at different places. The present instant really does not extend beyond this immediate point. At other points there are instants older, younger, and contemporaneous with this instant. They are, however, quite distinct from this one. In order for an instant to be simultaneous it would be necessary that it should occur at the same point. An object or event gains its substance and form from activities of our minds. Any meaning or significance that an object or event has is also derived from our minds. The reality of the universe is an activity, or series of activities, which are manifested in life and mind. The relativity of space is illustrated by an example given by Professor Henri Poincaré. Assume that I meet you in Wall Street, New York, and say, "I will meet you here again at this time to-morrow." You promise to do so. But you could not keep such a promise except with regard to position on the surface of the earth, because between now and to-morrow the earth will have moved over an enormous distance carrying Wall Street and a great mass of other things with it. The sun also will have moved away the stars, carrying the earth with it.