Prior reading of those great books which the author of a great book had read himself helps us in understanding a great book. For example, Euclid, the writer of the 'Elements of Geometry' needs no prior reading. Because this book is genuinely an introduction to geometry and basic arithmetic. But the same cannot be said of Newton's 'Mathematical Principles of Natural Philosophy'. Because Newton's applied mathematics is solving physical problems. Newton's style shows that he was highly influenced by proportions. This example of Newton shows that the prior reading of those readers which the writer of a great book had himself had read, helps us in reading a good book.