To Derive Two Or Three General Principles

A Vulgar Mechanick Can Practice What He Has Been Taught Or Seen Done, But If He Is In An Error He Knows Not How To Find It Out And Correct It, And If You Put Him Out Of His Road He Is At A Stand. Whereas He That Is Able To Reason Nimbly And Judiciously About Figure, Force, And Motion, Is Never At Rest Till He Gets Over Every Rub. (from A Letter Dated 25 May, 1694)

Thus Far I Have Explained The Phenomena Of The Heavens And Of Our Sea By The Force Of Gravity, But I Have Not Yet Assigned A Cause To Gravity. Indeed, This Force Arises From Some Cause That Penetrates As Far As The Centers Of The Sun And Planets Without Any Diminution Of Its Power To Act, And That Acts Not In Proportion To The Quantity Of The Surfaces Of The Particles On Which It Acts (as Mechanical Causes Are Wont To Do) But In Proportion To The Quantity Of Solid Matter, And Whose Action Is Extended Everywhere To Immense Distances, Always Decreasing As The Squares Of The Distances.

What Descartes Did Was A Good Step. You Have Added Much Several Ways, And Especially In Taking The Colours Of Thin Plates Into Philosophical Consideration. If I Have Seen A Little Further It Is By Standing On The Shoulders Of Giants.

We Are To Admit No More Causes Of Natural Things Than Such As Are Both True And Sufficient To Explain Their Appearances.

To Arrive At The Simplest Truth Requires Years Of Contemplation.