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An Analytic Perspective on Modern Philosophy
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The Philosophical Writings of Descartes: Volume 2The Philosophical Writings of Descartes: Volume 2 by Rene Descartes
Buy new: $27.29 / Used from: $9.48
Best edition. Notably includes the complete Objections and Replies. You may want to supplement with "Discourse" and "First Philosophy".
An Essay Concerning Human Understanding (Clarendon Edition of the Works of John Locke)An Essay Concerning Human Understanding (Clarendon Edition of the Works of John Locke) by John Locke
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The other side of the picture: Empiricism.
The Philosophy of LeibnizThe Philosophy of Leibniz by Bertrand Russell
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Russell tries to piece together Leibniz's difficult philosophical program.
G. W. Leibniz's MonadologyG. W. Leibniz's Monadology by Nicholas Rescher
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Eclectic but worth reading. It's impressive to know about it, anyway.
Principles of Human Knowledge and Three Dialogues Between Hylas and Phil (Penguin Classics)Principles of Human Knowledge and Three Dialogues Between Hylas and Phil (Penguin Classics) by George Berkeley
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Don't know about this edition, but the texts are important in the development of modern philosophy.
A Treatise of Human Nature (Oxford Philosophical Texts)A Treatise of Human Nature (Oxford Philosophical Texts) by David Hume
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Big treatise. The first "book" is considered the most important. The others can be gleaned from secondary sources.
An Enquiry concerning Human Understanding (Oxford Philosophical Texts)An Enquiry concerning Human Understanding (Oxford Philosophical Texts) by David Hume
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The "easier" version, with new problems and thoughts addressed. Hume is, obviously, a pretty important fella.
The Cambridge Companion to Hume (Cambridge Companions to Philosophy)The Cambridge Companion to Hume (Cambridge Companions to Philosophy)
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A substantive philosopher requires further discussion to be properly understood.
Critique of Pure Reason (The Cambridge Edition of the Works of Immanuel Kant in Translation)Critique of Pure Reason (The Cambridge Edition of the Works of Immanuel Kant in Translation) by Immanuel Kant
The end of an era. If ever there was a must-read book in the category of dusty-old-treatises, this is it. Addresses metaphysics and epistemology.
Practical Philosophy (The Cambridge Edition of the Works of Immanuel Kant in Translation)Practical Philosophy (The Cambridge Edition of the Works of Immanuel Kant in Translation) by Immanuel Kant
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The next critique (and other moral writings, but for $100, a bit pricey). The topic: ethics.
Critique of the Power of Judgment (The Cambridge Edition of the Works of Immanuel Kant in Translation)Critique of the Power of Judgment (The Cambridge Edition of the Works of Immanuel Kant in Translation) by Immanuel Kant
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Incidentally, these Cambridge editions are great, albeit expensive. The topic: aesthetics and teleology.
The Cambridge Companion to Kant and Modern Philosophy (Cambridge Companions to Philosophy)The Cambridge Companion to Kant and Modern Philosophy (Cambridge Companions to Philosophy)
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Useful analysis and essaying. If Hume deserves extra study, Kant certainly does, too!
A History of Western PhilosophyA History of Western Philosophy by Bertrand Russell
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Borderline useless as a history. But a lot of fun reading Russell's perspective on things.
Science of Logic (Muirhead Library of Philosophy)Science of Logic (Muirhead Library of Philosophy) by Georg Wilhelm Friedrich Hegel
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Useless! This path leads you toward continental philosophy.
The Frege Reader (Blackwell Readers)The Frege Reader (Blackwell Readers) by Gottlob Frege
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This path leads to analytic philosophy. Have a brain, follow the clearly marked, respectable, productive road.