Thus, in biology, we used to have interminable discussion as to whether certain single-celled organisms were animals or vegetables, until Haeckel introduced the new apperceptive name of Protista, which ended the disputes. In law courts no _tertium quid_ is recognized between insanity and sanity. If sane, a man is punished: if insane, acquitted; and it is seldom hard to find two experts who will take opposite views of his case. All the while, nature is more subtle than our doctors. Just as a room is neither dark nor light absolutely, but might be dark for a watchmaker's uses, and yet light enough to eat in or play in, so a man may be sane for some purposes and insane for others,--sane enough to be left at large, yet not sane enough to take care of his financial affairs. The word 'crank,' which became familiar at the time of Guiteau's trial, fulfilled the need of a _tertium quid_. The foreign terms 'desequilibre,' 'hereditary degenerate,' and 'psychopathic' subject, have arisen in response to the same need. The whole progress of our sciences goes on by the invention of newly forged technical names whereby to designate the newly remarked aspects of phenomena,--phenomena which could only be squeezed with violence into the pigeonholes of the earlier stock of conceptions. As time goes on, our vocabulary becomes thus ever more and more voluminous, having to keep up with the ever-growing multitude of our stock of apperceiving ideas. In this gradual process of interaction between the new and the old, not only is the new modified and determined by the particular sort of old which apperceives it, but the apperceiving mass, the old itself, is modified by the particular kind of new which it assimilates. Thus, to take the stock German example of the child brought up in a house where there are no tables but square ones, 'table' means for him a thing in which square corners are essential. But, if he goes to a house where there are round tables and still calls them tables, his apperceiving notion 'table' acquires immediately a wider inward content. In this way, our conceptions are constantly dropping characters once supposed essential, and including others once supposed inadmissible. The extension of the notion 'beast' to porpoises and whales, of the notion 'organism' to society, are familiar examples of what I mean.