6. The monadology has very important consequences in reference to THE THEORY OF KNOWLEDGE. As the philosophy of Leibnitz, by its opposition to Spinozism, had to do with the doctrine of being, so by its opposition to the empiricism of Locke must it expound the theory of knowledge. Locke’s Essay on the Human Understanding had attracted Leibnitz without satisfying him, and he therefore attempted a new investigation in his _Nouveaux Essais_, in which he defended the doctrine of innate ideas. But this hypothesis of innate ideas Leibnitz now freed from that defective view which had justified the objections of Locke. The innateness of the ideas must not be held as though they were explicitly and consciously contained in the mind, but rather the mind possesses them potentially and only virtually, though with the capacity to produce them out of itself. All thoughts are properly innate, _i. e._ they do not come into the mind from without, but are rather produced by it from itself. Any external influence upon the mind is inconceivable, it even needs nothing external for its sensations. While Locke had compared the mind to an unwritten piece of paper, Leibnitz likened it to a block of marble, in which the veins prefigure the form of the statue. Hence the common antithesis between rational and empirical knowledge disappears with Leibnitz in the degrees of greater or less distinctness.—Among these theoretically innate ideas, Leibnitz recognizes two of special prominence, which take the first rank as principles of all knowledge and all ratiocination,—the principle of contradiction (_principium contradictionis_), and the principle of sufficient cause (_principium rationis sufficientis_). To these, as a principle of the second rank, must be added the _principium indiscernibilium_, or the principle that there are in nature no two things wholly alike.