ON THE THEORY OF SCIENCE 337 



actually could be, since we have not been able to observe every 

 agreement, and our concept is always poorer in constituents at any 

 given time than it might be. To seek out such elements of concepts 

 as have been overlooked, and to prove that they are necessary factors 

 of the corresponding experiences, is one of the never-ending tasks 

 of science. The other case, namely, that elements have been received 

 in the concept which do not prove to be constant, also happens, and 

 leads to another task. One can then leave that element out of the 

 concept, if further experiences show that the other elements are 

 found in them, or one can form a new concept which contains the 

 former elements, leaving out those that have been recognized as 

 unessential. For a long time the white color belonged to the concept 

 swan. When the Dutch black swans became known, it was possible 

 either to drop the element white from the concept swan (as actually 

 happened), or to make a new concept for the bird which is similar 

 to the swan but black. Which choice is made in a given case is largely 

 arbitrary, and is determined by considerations of expediency. 



Into the formation of concepts, therefore, two factors are operat- 

 ive, an objective empirical factor, and a subjective or purposive 

 factor. The fitness of a concept is seen in relation to its purpose, 

 which we shall now consider. 



The purpose of a concept is its use for prediction. The old logic 

 set up the syllogism as the type of thought-activity, and its simplest 

 example is the well-known 



All men are mortal, 

 Caius is a man, 

 Therefore Caius is mortal. 



In general, the scheme runs 



To the concept M belongs the element B, 

 C belongs under the concept M, 

 Therefore the element B is found in C. 



One can say that this method of reasoning is in regular use even 

 to this day. It must be added, however, that this use is of a quite 

 different nature from that of the ancients. Whereas formerly the 

 setting up of the first proposition or the major premise was con- 

 sidered the most important thing, and the establishment of the 

 second proposition or minor premise was thought to be a rather 

 trifling matter, now the relation is reversed. The major premise con- 

 tains the description of a concept, the minor makes the assertion 

 that a certain thing belongs under this concept. What right exists 

 for such an assertion? The most palpable reply would be, since 

 all the elements of the concept M (including B) are found in C, C 

 belongs under the concept M. Such a conclusion would indeed be 

 binding, but at the same time quite worthless, for it only repeats the 



