268 THE EVOLUTION OF SCIENTIFIC THEORIES 



plausibility of its analogy or model. Copernicus' theory proposes to 

 explain the diurnal motion of the heavens using a model that repre- 

 sents man facing the heavens as one who in a moving ship observes 

 the "motion" of the shore. Owing to our unawareness of the motion 

 of our "ship" this model seems much less plausible than Ptolemy's, in 

 which the heavens move as does a ship seen by a stationary observer 

 on the shore. Lavoisier's new theory is at first criticized for the sheer 

 implausibility of its postulation of a host of elements when "every- 

 body knew" that there could be but one or a few fundamental ele- 

 ments. Newton's theory is rejected by Leibniz for the sheer absurdity 

 of what he took to be Newton's idea of a gravitational force acting 

 across the abyss of interplanetary space. The "obvious" difference 

 between thinking man and dull beast gave an air of implausibility to 

 Darwin's conception of evolution, and the marvelous organization of 

 living things to his concept of a random "natural selection." Who 

 could seriously credit Arrhenius' view that oppositely charged ions 

 are "spontaneously" separated from each other in solutions that draw 

 on no outside source of energy? 



Quite obviously explanatory appeal is a complex function of a 

 multitude of scientific, cosmologic, and cultural factors. Observ- 

 ing the profoundly subjective quality of such appeal, we may, if 

 Ave like, deplore its use as a criterion of scientific judgment. Yet, as 

 Ave saw earlier, the appeal of its mathematical harmony was very 

 nearly the sole support of the Copernican theory in its youth. In any 

 case, whether or not the criterion of appeal should be disregarded, 

 quite plainly it has not been disregarded. Scientific theories are hu- 

 manly judged by men never inhumanly unmindful of explanatory 

 appeal and the subjective considerations that determine it. To the 

 extent that judgment is affected by such appeal, we find here a major 

 bias toward conservatism. An established theory learned early in life 

 Avill generally have for us a plausibility no genuinely novel tlieory 

 can later rival. 



The two criteria of immediate judgment. Efficiency and appeal are 

 the only criteria always available for judgment of a totally new the- 

 ory. Ordinarily some time must pass before we can gauge its heuristic 

 power by the full range of discoveries to which it may or may not 

 lead. And have we not then reached a complete impasse? The 

 criterion of efficiency is often biased against novelty, the criterion of 

 appeal almost invariably so. Must we not then conclude diat a funda- 



