Abstract: The existential turning of modern western philosophy is not accidental but closely related to the westerners pursuit of ration. From the ancient Greece， the westerners set up the rational image of the lonely， which lost its original existential foundation with the emergence of science. The appearance of existentialism does not mean the loss of ration in the western society; it simply means that the way of inquiring into rational life has changed. Looking back on the occurrence and decline of ration， it is found that the internal logic of developing ration is to pursue a rational life which can be explained. With the appearance of existentialism， the westerners began to realize the pre-existence of real existence to rational existence and strove to reconstruct the path to rational life from reality.

Key words: God, idea, substance, ration, certainty