Leibniz-Clarke correspondence
http://www.newtonproject.sussex.ac.uk/catalogue/viewcat.php?id=THEM00224
Hume’s Enquiry Concerning Human Understanding
http://davidhume.org/texts/ehu.html
Shepherd’s An Essay upon the Relation of Cause and Effect
https://archive.org/details/essayuponrelatio00shepialahttps://archive.org/details/essayuponrelatio00shepiala
Stewart Duncan 