A problem which is discussed by many philosophers of this period and subsequently is the problem of free will and determinism. The rise of Newtonian science, for example, led to the belief that the natural world is a deterministic system. But the pre
ssures of determinism can clearly be seen as early as Descartes and Hobbes. What about free will then? Descartes, Locke, Hume, Holbach and others have discussions of this issue. Probably it would be best to pick one such author and ask:
(a) Is Holbach's (or Hobbes) system of determinism consistent or plausible?
(b) Is Hume or Kant's solution to the problem of free-will and determinism satisfactory?
(c) How does accepting determinism effect one's views about the problem of evil? Jonathan Edwards or possibly Anthony Collins would be good candidates for research here.
A good short account of determinism by a contemporary hard determinist is Richard Taylor's chapters on hard determinism and fatalism in his book Metaphysics, Prentice Hall. Hume's position can be found in the Inquiry Concerning Human Understanding and Treatise on Human Nature. Leibniz is of particular interest in regard to this question. I would start with the MacDonal d book Leibniz. An American philosopher who was influenced by Locke and others is Jonathan Edwards, a New England Calvinist preacher. A contemporary treatment of this problem is found in Daniel Dennett's book Elbow Room -- Dennett is a defender of free will.