The disconcerting aspect of the situation resides in the difficulty with which mankind throws off beliefs that have be- come habitual. The test of ideas, of thinking generally, is found in the consequences of the acts to which the ideas lead, that is in the new arrangements of things which are brought into existence. Such is the unequivocal evidence as to the worth of ideas which is derived from observing their position and role in experimental knowing. But tradition makes the tests of ideas to be their agreement with some antecedent state of things. This change of outlook and standard from what pre- cedes to what comes after, from the retrospective to the pros- pective, from antecedents to consequences, is extremely hard to accomplish. Hence when the physical sciences describe ob- jects and the world as being such and such, it is thought that