This presentation of the problem is perhaps not calculated to inspire excitement or let loose agitating passions; and apparently the end of the nineteenth century will not witness the renewal of the philosophical debates on the subject which characterised the last half of the eighteenth. Never either, before or since, has there been so much agitation, nor have men’s minds been so tossed by diverse currents. Many various theories were promulgated at the time, but opinions grouped themselves chiefly round two diametrically opposite schools of thought, towards one or the other of which they leaned. According to Hume, Condillac and their adherents, matter alone exists; our understanding, our feelings, our will are only transformed sensations. This was pure materialism. Pure idealism was represented by Berkeley, who went so far as to deny the reality of matter; according to him the bodies making up the universe have no real existence; the true realities were God and the ideas He produced in us. Those who preserved their ancient beliefs were the most troubled, they began to ask themselves whether the foundations of their faith were solid, and they much desired to see certain problems solved. These thoughts had exercised the minds of the sages of India, the thinkers of Greece, the dreamers of Alexandria, and the divines and scholars of the Middle Ages. They were the old problems of the world, what we know of the Infinite, the questions of the beginning and end of our existence; the questions of the possibility of absolute certainty in the evidence of the senses, of reason or of faith. How much was comprehended in these enquiries. One hundred years previously, the cautious reasoner, Descartes, instead of asking “What do we know?” posed in its place the question, “How do we know?” This was in fact a fundamental question which appealed to philosophers who followed Descartes, as of the utmost importance, and they also asked themselves, “After what manner does the human mind acquire what it knows?” What is called Locke’s tenet, “Nihil est in intellectu quod non ante fuerit in sensu,” Leibnitz answered by “Nihil--nisi intellectus.” Noiré gives this sentiment a fresh turn by saying: “There is nothing in this plant that was not already in the soil, the water and the atmosphere, but that which causes this plant to be a plant.” Condillac, who agreed with Locke, thus formulated his opinion: “Penser c’est sentir”; or, “In order to feel it is necessary to possess senses,” which is self-evident.