We may put the point in another way by asking by what right the mind can prescribe to or anticipate experience. Kant’s answer is just in so far as we can determine the conditions under which alone objects can be known. If that can be done, we can say, These principles will hold of objects in so far as they are known. In the preface to the second edition of the _Critique of Pure Reason_ Kant reverts to the discoveries of Galileo and Torricelli, and points out that their success was due to their asking of nature the right question, and the right question was that which reason could understand. "When Galileo let balls of a particular weight, which he had determined himself, roll down an inclined plane, or Torricelli made the air carry a weight, which he had previously determined to be equal to that of a definite volume of water, a new light flashed on all students of nature. They comprehended that reason has insight into that only which she herself produces on her own plan, and that she must move forward with the principles of her judgments, according to fixed law, and compel nature to answer her questions, but not let herself be led by nature, as it were in leading-strings. Otherwise accidental observations, made on no previously fixed plan, will never converge towards a necessary law, which is the only thing that reason seeks or requires. Reason, holding in one hand its principles, according to which alone concordant phenomena can be admitted as laws of nature, and in the other the experiment which it has devised according to those principles, must approach nature in order to be taught by it, but not in the character of a pupil who agrees to everything the master likes, but as an appointed judge, who compels the witnesses to answer the questions which he himself proposes." Kant, here, is concerned with reason in its application to experience, and he makes it clear that there is much in all such inquiries which cannot be anticipated a priori. "Reason must approach nature in order to be taught by it." The answer to the questions and experiments cannot be known beforehand. The empirical element in science cannot be explained away. Reason dictates not the answer but the question, and so far the form of the answer. Reason, then, it is suggested, is concerned with the principles or conditions, according to which we can understand things. It is not a method of observing or analysing objects; rather it states the methods and principles according to which objects must be observed if they are to be understood. The principles are not statements about the nature of objects, but principles of the possibility of experience. This new attitude to reason Kant describes as the Copernican change in philosophy. It constitutes Kant’s idealism. Its nature and importance we must examine in the next chapter. *CHAPTER III* *KANT’S IDEALISM. TIME AND SPACE*