The accurate and discriminating observation of facts, supported by careful experiment, is certainly a great advantage that modern science has over all earlier efforts to attain the truth. The distinguished thinkers of classic antiquity were far superior to most modern scientists and philosophers in regard to judgment and reasoning, or all the subtler processes of thought; but they were superficial and unpractised observers, and were barely acquainted with experiment. In the Middle Ages scientific work degenerated in both its aspects, as the dominant creed demanded only faith and the recognition of its supernatural revelation, and depreciated observation. The great importance of this as a foundation of real knowledge was first appreciated by Bacon of Verulam, whose _Novum Organon_ (1620) laid down the principles of scientific knowledge, in opposition to the current scholasticism derived from Aristotle and his _Organon_. Bacon became the founder of modern empirical investigation, not only by making careful and exact observation of phenomena the basis of all philosophy, but also in demanding the supplementing of this by experiment; by this experiment he understood the putting of a question to Nature, as it were, which she must herself answer--a kind of observation under definite and deliberate conditions. This more rigorous method of "exact observation," which is hardly three hundred years old, was very strongly aided by the inventions which enable the human eye to penetrate into the farthest abysses of space and the profoundest depths of smaller bodies--the telescope and microscope. The great improvement in these instruments during the nineteenth century, and the support given by other recent inventions, have led to triumphs of observation in this "century of science" that surpassed all anticipation. However, this very refinement of the technique of observation has its drawbacks, and has led to many an error. The effort to obtain the utmost accuracy in _objective_ observation has often led to a neglect of the part which is played by the _subjective_ mental action of the observer; his judgment and reason have been depreciated in comparison with the acuteness and clearness of his vision. Frequently the means has been turned into the end of knowledge. In the reproduction of the thing observed the objective photograph, presenting all parts of the object with equal plainness, has been more valued than the subjective design that reproduces only what is essential and leaves out what is superfluous; yet the latter is in many cases (for instance, in histological observation) much more important and correct than the former. But the greatest fault has been that many of these "exact" observers have refrained altogether from reflection and judgment on the phenomena observed, and have neglected subjective criticism; hence it is that so often a number of observers of the same phenomenon contradict each other, while each one boasts of