PHILOSOPHICAL AND PSYCHOLOGICAL ASPECTS 235 



there exists among competent scientists, a consensus of 

 opinion which is formally known as the cell-theory; in other 

 words a common sense in which this phrase is accepted. The 

 only way this sense differs from the sense of persons without 

 biological experience is that it rests upon wider and more 

 critical observation and is, therefore, the more reliable. It 

 happens that these conclusions regarding cells may be drawn 

 only by persons trained to the use of microscopes ; and only 

 after special preparation of the materials examined, which 

 is an illustration of what Huxley meant by trained and or- 

 ganized sense. It is not that the observations and conclu- 

 sions involved are fundamentally different from those of 

 everyday life. They are refinements of these, made pos- 

 sible by the training of the scientist and the organization 

 of his material. There is no necromancy in science. Its 

 methods are the logical methods of thought which normal 

 individuals regularly use. Science has often made initial 

 strides, through the work of investigators who perceived 

 the unifying features in large groups of previously unrelated 

 phenomena, and whose daring hypotheses at first resembled 

 the flight of poetic imagination or the vision of some genius 

 of the commercial world. But what has finally counted has 

 been the confirmation of hypotheses step by step, until they 

 have become commonplace knowledge verifiable by any- 

 one who reviews the phenomena. 



Another example may be given. Certain of the early 

 embryologists defended the dictum, omne vivum ex ovo, as 

 expressing the manner of generation; and later embryolo- 

 gists have extended this generalization, until we accept the 

 statement that " every cell comes from a preexisting cell." 

 We mean by the modern statement of the older doctrine, 

 that the facts have been recorded by earlier investigators 

 and confirmed by later ones ; that we have seen for ourselves 

 the process of fertilization and development; and that our 

 fellow workers are familiar with the phenomena, for they 

 talk with us of what they have seen. Moreover, it is assumed 



