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1. Hume's Influences

In a 1737 letter, Hume wrote that readers of the Treatise would benefit by looking at writings by Nicolas Malebranche, George Berkeley, Pierre Bayle, and Renй Descartes:

I shall submit all my Performances to your Examination, & to make you enter into them more easily, I desire of you, if you have Leizure, to read once over le Recherche de la Veritй of Pere Malebranche, the Principles of Human Knowledge by Dr Berkeley, some of the more metaphysical Articles of Baile's Dictionary; such as those of Zeno, & Spinoza. Des-Cartes Meditations wou'd also be useful, but I don't know if you will find it easily among your Acquaintances. These Books will make you easily comprehend the metaphysical Parts of my Reasoning. And as to the rest, they have so little Dependence on on [sic] all former Systems of Philosophy, that your natural Good Sense will afford you Light enough to judge of their Force & Solidity. [Hume to Michael Ramsay, August 26, 1737]

Chronologically, the first philosopher on Hume's list is Renй Descartes (1596-1650). In his Meditations on the First Philosophy (1641), Descartes combats sceptics who doubt the existence of the external world and the reliability of our senses. To accomplish his task, Descartes himself provisionally plays the role of a sceptic and doubts everything that can possibly be doubted. Descartes then arrives at one absolute truth - his own existence - and uses this as a foundation for demonstrating all knowledge. Hume was probably influenced by Descartes's provisional doubting process, as Hume himself doubted the sources of human knowledge. Throughout Hume's philosophical writings, though, he also reacted against the more speculative metaphysical views that Descartes developed.

The remaining three philosophers listed in Hume's letter - Malebranche, Bayle, and Berkeley - were controversial figures when their writings first appeared, and they share the conviction that the true nature of the world is not as evident as we ordinarily think. French Catholic philosopher Nicolas Malebranche (1638-1715) was a follower of Descartes and is most remembered for his Search After Truth (1674-1675). Two themes stand out in that work, both of which influenced Hume. First, Malebranche wrestled with how our minds receive perceptual images from external objects. For example, as I stand in front of a tree, I have a visual image of that tree. How does the tree itself cause that image in my mind? For Malebranche, the tree is physical in nature, yet my perceptual image is spiritual in nature, and, so, something like a miracle must take place to convert the one to the other. After rejecting various theories of perception, Malebranche concludes that God possesses mental/spiritual images of all external things, and that he implants these ideas in our minds at the appropriate time - when I stand before the tree, for example. In short, according to Malebranche, we see external objects by viewing their images as they reside in God. Hume did not adopt Malebranche's theological solution to this problem, but perhaps Hume learned from Malebranche that there is a great gulf between external objects and our perceptions of them, and that it is exceedingly difficult to explain the connection between the two.

The second major theme in Malebranche concerns the nature of causality, or, more specifically, how two events (such as the motion of a stick that strikes and moves a ball) are causally connected. Malebranche argues that physical objects by themselves simply cannot be the cause of motion in other objects; only spirits can do that. So, when a stick strikes a ball, some spiritual force must intervene and actually cause the ball to move. Malebranche concludes that God is the true cause of the ball's motion, and that the movement of the stick is only the occasion, the occasional cause, of the ball's motion. Malebranche pushes this theory further and argues that God is also the true cause behind human bodily motion. For example, when I wilfully pick up a book, my will is only the occasional cause, and God is the true cause. Again, Hume did not adopt Malebranche's theological solution to the problem of causality, but it was perhaps through Malebranche that Hume became aware of the difficulty of explaining the nature of causal connections.

Although Malebranche raised questions about our knowledge of external objects and causality, he was nevertheless optimistic about the ability of our human reason to unravel these philosophical mysteries. However, influenced by the ancient Greek sceptical traditions, French philosopher Pierre Bayle (1647-1706) was much more pessimistic about our rational abilities. A philosophy and history professor, Bayle made a lasting mark in philosophy with his monumental Historical and Critical Dictionary (Dictionaire historique et critique, 1692). The Dictionary contains hundreds of articles on notable figures from ancient through modern times, and in lengthy footnotes to these articles Bayle presents his own original and often radical views. In the letter cited above, Hume mentions "the more metaphysical Articles of Baile's Dictionary" and cites two particular articles: Zeno of Elea and Spinoza.

Zeno (. c. 450 BCE) was a follower of Greek philosopher Parmenides and, like his teacher, Zeno argued that our ordinary notions of the world are illusions. Zeno presents a series of logical paradoxes that show the inherently contradictory nature of motion and space. Bayle comments extensively on Zeno's paradoxes and suggests that space is composed of one of three possible things: mathematical points, indivisible physical points, or infinitely divisible parts. Bayle sceptically concludes that all three of these views are absurd, and, so, no adequate explanation of space is possible. Drawing on Bayle's discussion, Hume concludes with an almost equally sceptical assessment regarding our notions of space and time. Benedict de Spinoza (1632-1677) - another controversial modern philosopher - argued that God is the single substance of the entire universe. What appear to be individual objects, such as rocks and trees, are in fact only modifications of God's single-substance. Bayle treats Spinoza contemptuously and argues that it is counterintuitive to see all physical things as modifications of a single substance. In the Treatise Hume extends Bayle's critique further and argues that theologians are equally counterintuitive when they say, for example, that my diverse mental images are really unified in my single spirit-mind.

A third metaphysical article in Bayle that certainly had an impact on Hume is that on Pyrrho (c. 365-c. 275 BCE). Pyrrho was the founder of one of the Greek sceptical traditions,

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