As the strength of Samson lay in his unshorn hair, so the strength of the mechanical theory lay, and still lies, in the acceptance of Locke's theory of human knowledge, i.e. that it is all derived from the senses. And the Delilah who can shear away Locke's conclusions, leaves Samson helpless; mechanical materialism becomes a discredited theory. Hence the truth of the saying that the problem of knowledge is the preliminary question for philosophy. WEAKNESS OF SPECULATIVE PHILOSOPHY.--Spinoza and Leibniz may be said to have dispensed with this foundation. Taking the scientific knowledge of their time for granted, they drew certain conclusions therefrom; but their results, however imposing, were felt to be the result rather of speculation than of reason. Such was the more or less unexpressed estimate of their work. It was undervalued, for both Spinoza and Leibniz were thinkers of the first calibre; and yet there was some justice in the charge. By the end of the eighteenth century the days of merely speculative philosophy were past. THE CRITICAL PHILOSOPHY.--The time was ripe for a new metaphysic--for a fresh step forward in philosophic method. That step was taken by the celebrated Immanuel Kant, who is the originator of what is known, in the history of thought, as the Critical Philosophy. The word _critical_ signifies a particular method of approaching the problem of existence, a method which must be contrasted with that of the _speculative_ philosophy, of which Spinoza and Leibniz are examples. The critical philosophy, before attempting (as Spinoza had done) to tackle the problem of _existence_, first attacked the problem of _knowledge_. Before asking _What is the truth?_ it put the preliminary question, _What are the means at man's disposal for reaching the truth?_ It prefaced all philosophical enquiry by an examination into the nature and scope of human thought. Such was the preparatory investigation which was to place metaphysics upon a secure and scientific foundation. For the new philosophy, the gateway to all sound knowledge is the reflection of the human mind upon itself. "Know thyself," is its advice to the inquiring spirit of man. Here, if anywhere, is to be found the philosopher's stone. IMMANUEL KANT.--The celebrated Immanuel Kant was born at Koenigsberg in 1724, and died in his native town in 1804. Between those dates he lived the industrious and uneventful life of a university professor. The Seven Years' War and the French Revolution left him undisturbed, though not unmoved. He was a man of quiet, regular habits, and his fellow-townsmen would set their clocks by his daily promenade.[23] But the adventurous originality of his thought serves as a contrast to this peaceful picture.