does not exist for us, because we cannot go back to it or ahead into it. But assume time were reversible, like space--that is, we could go through it in any direction. There would then be no such thing as birth or origin, and death or extinction, but our life would exist permanently, as a part or span of time, just as the house exists as a part or section of space, and the question of immortality, of extinction or nonextinction by death, would then be meaningless. We should not exist outside of the span of time covered by our life, just as we do not exist outside of the part of space covered by our body in space, and to reach an event, as our life, we should have to go to the part of space and to the part of time where it occurs; but there would be no more extinction of the life by going beyond its length in time as there is extinction of a house by going outside of its door, and everything, like a human being, would have four extensions or dimensions--three extensions in space and one in time.[9] If space and time, and therefore the characteristics of space and time, are not real things or entities, but conceptions of the human mind, then those transcendental questions, as that of immortality after death and existence before birth, are not problems of fact in nature or outside of nature, but are meaningless, just as the question whether a house exists for an observer outside of the space covered by it. In other words, the questions of birth and death, of extinction or immortality, are merely the incidental results of the peculiarity of our conceptions of time, the peculiarity that the time of our conceptions is irreversible, flows continuously at a uniform rate in the same direction from the past to the future. But if time has no reality, is not an existing entity, then these transcendental problems resulting from our time conception, of extinction or immortality, have no real existence, but are really phenomena of the human mind, and cease to exist if we go beyond the limitations of our mind, beyond our peculiar time conception. It is interesting to realize that the modern development of science, in the relativity theory, has proved not only that time is not real, but a conception, but also has proved that the time of our conception does not flow uniformly at constant rate from past to future, but that the rate of the flow of time varies with the conditions; the rate of time flow of an event slows down with the motion relative to the event. But the conception of a reversal of the flow of time is no more illogical than the conception of a change of the rate of the flow of time. It is inconceivable, because it is beyond the limitations of our mind. Thus we see that the questions of life and death, of extinction and immortality, are not absolute problems, but merely the result of the limitations of our mind in its conception of time, and have no existence outside of us.