Traditional textbooks on the history of philosophy have generalized modern philosophy as an epistemological philosophy.However, how to understand a metaphysical pursuit of modern rationalists in a pure epistemological frame? A profound problem met by rationalists is: how to insure freedom of human will at the same time of accepting a mechanistic world view?To solve the problem, both Spinoza and Leibniz choose to throw away absolute objectivity of space, debasing the extensional world into a phenomenal world presented to human knowledge which has just a kind of secondary objectivity.As a result, man is able to surpass the necessary chain of causality of the phenomenal world so as to arrive at true freedom.Finally, Kant exhibits this puzzlement, and efforts of Spinoza and Leibniz are realized in Kant's philosophy in a perfect way.