1-5. Plato, the EUTHYPHRO, APOLOGY, and CRITO [all short, pretty much a quick read], the GORGIAS [my all time favorite dialogue], the REPUBLIC, the THEATETUS, maybe the SOPHIST. I consider this five books, not seven. The first three are really one book.
6-7. Aristotle, METAPHYSICS, PHYSICS, NICHOMACHEAN ETHICS.
8. Medieval Philosophy -- I don't know. You can't read the entire SUMMA by Aquinas, God knows. But you need somehow to familiarize yourselves with the metaphysical debates of the Middle Ages [which includes the important Jewish and Arabic philosophers].
9-10. Descartes, DISCOURSE ON METHOD and MEDITATIONS. With the MEDITATIONS, it is fascinating to read around in the volume of Responses that Descartes got when he sent it out to all the important philosophers in Europe [there is a funny story with that -- remind me to tell you.] The fascinating thing is that most of the objections that four centuries of philosophers have thought up to the MEDITATIONS appear in those responses, which were written within weeks of receiving the work.
11. Leibniz, MONADOLOGY
[Maybe -- Spinoza, ETHICS. Maybe not.]
12. Thomas Hobbes, LEVIATHAN
13-14. John Locke, ESSAY CONCERNING THE HUMAN UNDERSTANDING, SECOND TREATISE ON CIVIL GOVERNMENT [the FIRST TREATISE is a hoot, but it is not necessary.]
15. George Berkeley, either A TREATISE CONCERNING THE PRINCIPLES OF HUMAN KNOWLEDGE or THREE DIALOGUES BETWEEN HYLAS AND PHOLINUS. Berkeley published both of these in his middle twenties! When I was a student, we had a game called "I am now older than *** when he wrote ***.] Berkeley and Hume were killers. Locke and Kant were reassuring. There was still plenty of time.
16-18. David Hume, A TREATISE OF HUMAN KNOWLEDGE, ENQUIRY CONCERNING THE HUMAN UNDERSTANDING, and DIALOGUES CONCERNING NATURAL RELIGION.
19. Jean-Jacques Rousseau, OF THE SOCIAL CONTRACT. [When I went to Oxford in 1954 as a twenty-year old graduate student on a traveling fellowship, the Kant scholar T. D. Weldon, who, I later learned, was more or less permanently drunk, told me I had to read ḖMILE. It was not good advice.]
20-22. Immanuel Kant, CRITIQUE OF PURE REASON, FOUNDATIONS OF THE METAPHYSICS OF MORALS, CRITIQUE OF JUDGMENT
[I pass in silence over Hegel]
23. Jeremy Bentham, INTRODUCTION TO THE PRINCIPLES OF MORALS AND LEGISLATION [If you are going to study Utilitarianism, you might as well know where it came from.]
24-25. John Stuart Mill, UTILITARIANISM, ON LIBERTY
At this point, everything explodes, with Kierkegaard [one of my favorite philosophers], Nietzsche, Frege, and all manner of other big names cropping up. I leave all of that to your professors.
And there it is! Are there other great books that I have omitted? Good lord, yes. Political Scientists are all gaga over Machiavelli, but I can't see it. Should you read Augustine's CITY OF GOD? Of course, but there are limits. "Twenty-five" has a nice ring to it, and if you get through all of the books on this list, you will have a better education than your fellow students [and very possibly than some of your professors, but don't tell them I said that.]
The really great thing about this list is that it has not changed since I was a student, and it will not have changed, I warrant, by the time you are my age! That is a claim that cannot be made in many other academic disciplines.