Human knowledge has two different phases-the pre-Einstein period and the post-Einstein period. In the pre-Einstein period, knowledge was confined to the macro or material world, which was observable and measurable. So, it was generally held that everything, which has a real existence, should also be observable. Anything, which could not be observed, had no real existence. This meant that only the seen world was real and what was unseen was unreal or some kind of fiction. This concept created the theory that is generally called logical positivism. It means that the only valid logical argument is one that is demonstrable in material terms; otherwise it is simply a baseless claim, and not a valid argument. But, in the post-Einstein period, in the early years of the 20th century, when the atom was split, the whole situation changed. After the splitting of the atom, matter as a solid substance, disappeared. It was replaced by the micro-world, beyond the atomic world, where everything was reduced to unseen waves - neither measurable nor observable. After this revolution in knowledge, logical or rational argument also changed drastically. This changing situation compelled the philosophers and the scientists to revise logical criteria. It has now become an accepted fact that inferential argument is as valid as direct argument. Present-day science includes so many things, such as electrons, the law of gravity, x-rays, etc., all of which are non-material in nature. They cannot be observed, but every scientist believes in their existence, for the simple reason that, although we cannot see these things directly, we can see their effect. For example, a falling apple, in the case of gravity, and a photofilm, in the case of x-rays. We believe in the existence of all these things, not by observation but by their result, in other words, by way of indirect knowledge or inferential argument. This change in human knowledge also changed the theory of logic. Now it is well established in science that inferential argument is as valid as direct argument. (For details, see Human Knowledge, by Bertrand Russell) In the pre-Einstein era, unbelievers held that the concept of God pertains to the unseen world. And since no direct argument was available to bear this out, belief in God was held to be illogical and all the relevant indirect arguments were considered scientifically invalid, since they were inferential in nature. But now the whole situation has changed. Nothing is observable. So the existence of anything can be established only by means of inferential argument, rather than by direct argument. If inferential argument is valid with regard to the unseen micro-world, it is also valid with regard to the existence of God. Bertrand Russell, although an atheist, in his book, "Why I am not a Christian", has admitted this fact. He says that the argument centering on design, propounded by theologians to prove the existence of God, is scientifically valid. Since ancient days, theologians have argued that when there is a design there must also be a designer. As we see that our world is well designed, it compels us to believe that there is a designer - that is God.