1-The Spirit of Laws
Charles de Montesquieu
http://www.constitution.org/cm/sol-02.htm

2-The Advancement of Learning
Francis Bacon
http://darkwing.uoregon.edu/~rbear/adv1.htm

3-THE ESSAYS

Francis Bacon
http://www.esp.org/books/bacon/essays/html/title3.html

4-THE NEW ORGANON

(TRUE DIRECTIONS CONCERNING THE INTERPRETATION OF NATURE)
Francis Bacon
http://www.constitution.org/bacon/nov_org.htm