The methods of the new natural science had already been practiced for a few centuries before the new theory was formulated. It first found vent in the expression of surprise at the great confidence with which men assumed to predict certain phenomena and to point out their connections. Our experience is limited to a few successive observations of the regularity or coincidence of events. But we attribute to natural laws, in which are expressed causal relations of phenomena, a general and necessary applicability which far exceeds our experience. The English thinker Hume was the first who clearly expressed and formulated the question--since called the problem of causality--why men always act in this manner. But as he believed the reason for such action should be sought in the nature of experience alone, experience being the only source of knowledge, and as he did not further investigate the special and distinct part played by the nature of the human mind in this experiential connection, he could not find any satisfactory answer. Kant, who made the first important step toward the solution of this question, had been trained in the school of rationalism which then dominated in Germany and which represented an adaptation of mediaeval scholasticism to the requirements of increased knowledge. Starting from the thesis that things which are logical in the mind must be real in nature, the rationalists formulated by mere deduction general truths about god, infinity and immortality. Under the influence of Hume, Kant became the critic of rationalism and thus the reformer of philosophy. The question, how it is that we have knowledge of generally applicable laws in which we have implicit confidence--such as mathematical theses, or the maxim that every change has a cause--was answered by Kant in this way: Experience and science are as much conditioned on properties inherent in the organization of our mind as on the impressions of the outer world. The former properties must necessarily be contained in all experience and science. Therefore everything dependent on this common mental part of science must be perfectly certain and independent of special sense impressions. Common to all experience, and inseparable from it, are the pure sense-conceptions (reine Anschauungsformen), such as space and time, while the many experiences, in order to succeed in forming understanding and science, must be connected by the pure mind-conceptions (reine Verstandesbegriffe), the so-called categories; among the latter also belongs causality.