But there is a fundamental difference between our space conception and our time conception, in that we can pass through time only in one direction, from the past to the future, while we can pass through space in any direction, from north to south, as well as from south to north--that is, time is irreversible, flows uniformly in one direction, while space is reversible, can be traversed in any direction. This means that when we enter a thing in space, as a house, we can approach it, pass through it, leave it, come back to it, and the thing therefore appears permanent to us, and we know, even when we have left the house and do not see it any more, that it still exists, and that we can go back to it again and enter it. Not so with time. On approaching a thing in time, an event such as a human life, it extends from a point in time--birth--over a length of time--the life--to an end point in time--death--just as the house in space extends from a point in space--say the north wall--over a length of space--its extent--to an end point in space--say the south wall. But when we pass beyond the end point of an event in time--the death of a life--we cannot go back to the event any more; the event has ceased, ended, the life is extinct. But let us imagine that the same irreversibility applied to the conception of space--that is, that we could move through space only from north to south, and not in the opposite direction. Then a thing in space, as a house, would not exist for us until we approached it. When we were approaching it, it would first appear indistinctly, and more and more distinctly the nearer we approached it, just as an event in time does not exist until we reach the point of its beginning, but may appear in anticipation, in time perspective, when we approach it, the more distinctly, the closer we approach it, until we reach the threshold of the time span covered by the event, and the event begins to exist, the life is born. So to us, if we could move only from north to south, the house would begin to exist only when we reached its north door. That point would be the "birth" of the house. Passing through the span of space covered by the house--this would for us be its existence, its "life," and when we stepped out of the south door the house would cease to exist for us, we could never enter it and turn back to it again--that is, it would be dead and extinct, just as the life when we pass beyond its end point in time. Thus birth and death, appearance and extinction of an event in time, as our life, are the same as the beginning and end point of a thing in space, like a house. But the house appears to us to exist permanently, whether we are in it, within the length between beginning and end point, or not; while the event in time, our life, appears to us to exist only during the length of time when we are between its beginning and its end point in time, and before and after it