Mathematics is, I believe, the chief source of the belief in eternal and exact truth, 

as well as the super-sensible intelligible world. 

 Geometry deals with exact circles, but no sensible object is exactly circular; 

however carefully we may use our compasses, there will be some imperfections and irregularities. 

 This suggests the view that all exact reasoning applies to ideal as opposed to sensible objects; 

it is natural to go further and to argue that thought is nobler than sense, 

and the objects of thought are more real than those of sense-perception.