46o THE SCIENTIFIC MONTHLY
logical meaning does not do this. The inanlar mother whom we invoked in imagination may be supposed to teach her child fonnal logic, and, in so doing, to make use of herself and her child to illustrate the logician's use of the terms genus and species. She might say to the child :
Yon and I are natural bodies like tbe rocks and the clouds; but since we talk with each other, a thing which neither rocke nor clouds can do, we are par- ticular kinds of natural bodies. When bodies etand in such relation as this to one another, we, as logicians, speak of them as being in the relation of genus and Bpeeies.
So far as I can see, this example, if supplemented by others of like import that might be drawn from inanimate nature, could be made to completely satisfy the needs of formal logic as touching its doc- trines of naming, defining, dividing, classifying. In a word, formal logic is not obliged to take cognizance of the fact that living nature contains any organisms other than man himself. Logic is something that can be used upon living beings generally with great effecV'-some- thing that can occupy itself very interestingly and profitably with such things, but it is not obliged to be so used.
Logic goes to nature to get illustrations of how thought works rather than to actually learn nature. Beverting to Jevons's statement that logic may be defined as the theory of classification, we may remark that, so far as external nature is concerned, while logic may be defined as the theory of classification, it can not be defined as the practise of classification. It is important to call attention to this distinction be- tween logic and biology since even biologists frequently fail to recognize it and are beguiled into trying to impose the laws of thought upon nature by asserting that such and such a supposition about nature is a " logical necessity . Although logic is so important to the natural scien- tist as an instrument, quite as important is it never to forget that it is only an instrument. Logic is one of the many children of nature; it is not its parent or ruler.
A practical point to be noticed here is that right regard for logic in the business of the taxonomist clearly reveals both the unwarrant- ableness and misfortune of the view, so widely held, that synoptic descriptions and classifications are artificial or puerile, and devoid of scientific value. If such a definition of man as that just given does not express his nature — ^is not a natural definition — in what terms, pray, can he be naturally defined? The definition is natural, but meager. This and not its artificiality is its fault; and from this fault arises the need for the second kind of classification spoken of at the outset.
To this other sort of classification and the second meaning of the word man, we now turn. Logic lays great stress on the difference between extension and intension in the meaning of names. When the
�� �
