When one passes from knowledge considered as a body of facts ascertained and available for use to the thing we call intellectual aptitude or culture--namely, the power of turning knowledge to account and of producing results in spheres other than material--and when we inquire whether mankind has made a parallel advance in this direction, it becomes necessary to distinguish three different kinds of intellectual capacity. The first may be called the power of using scientific methods for investigating phenomena, whether physical or social. [Sidenote: No Decrease of Knowledge is now Likely] The second is the power of speculation, applied to matters which have not hitherto been found capable of examination by the methods of science, whether observational, experimental, or mathematical. The third is the power of intellectual creation, whether literary or artistic. The methods of scientific inquiry may almost be classed with the ascertained facts of science or with inventions, as being parts of the stock of accumulated knowledge built up by the labour of many generations. They are known to everybody who cares to study them, and can be learnt and applied by everybody who will give due diligence. Just as every man can be taught to fire a gun, or steer a ship, or write a letter, though guns, helms, and letters are the result of discoveries made by exceptionally gifted men, so every graduate in science of a university can use the methods of induction, can observe and experiment with a correctness which a few centuries ago even the most vigorous minds could scarcely have reached. [Sidenote: Original Thinkers are still Rare]