160 Circulation of the Blood 



agree with those that are perceived by the senses ; 

 when the things have been thoroughly scrutinized, then 

 are the senses to be trusted rather than the reason." 

 Whence it is our duty to approve or disapprove, to 

 receive or reject everything only after the most careful 

 examination ; but to examine, to test whether anything 

 have been well or ill advanced, to ascertain whether 

 some falsehood does not lurk under a proposition, it 

 is imperative on us to bring it to the proof of sense, 

 and to admit or reject it on the decision of sense. 

 Whence Plato in his Critias says, that the explanation 

 of those things is not difficult of which we can have 

 experience ; whilst they are not of apt scientific appre- 

 hension who have no experience. 



How difficult is it to teach those who have no 

 experience, the things of which they have not any 

 knowledge by their senses ! And how useless and 

 intractable, and unimpregnable to true science are such 

 auditors ! They show the judgment of the blind in 

 regard to colours, of the deaf in reference to concords. 

 Who ever pretended to teach the ebb and flow of the 

 tide, or from a diagram to demonstrate the measure- 

 ments of the angles and the proportions of the sides 

 of a triangle to a blind man, or to one who had 

 never seen the sea nor a diagram ? He who is not 

 conversant with anatomy, inasmuch as he forms no 

 conception of the subject from the evidence of his 

 own eyes, is virtually blind to all that concerns 

 anatomy, and unfit to appreciate what is founded 

 thereon ; he knows nothing of that which occupies 

 the attention of the anatomist, nor of the principles 

 inherent in the nature of the things which guide him 

 in his reasonings ; facts and inferences as well as their 

 sources are alike unknown to such a one. But no 

 kind of science can possibly flow, save from some 

 pre-existing knowledge of more obvious things ; and 

 this is one main reason why our science in regard to 

 the nature of celestial bodies, is so uncertain and 



