The BBC is putting it to a vote. Ed Feser has weighed in on the question with a balanced essay with which I largely agree. Feser gives the palm to Thomas Aquinas. Despite my deep respect for the doctor angelicus, I do not think he deserves the top position in the pantheon.
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Feser mentions five criteria: depth and centrality of the problems addressed; breadth of vision; originality; influence; truth of the ideas promulgated. I would add a sixth: belief in the power of discursive reason to arrive at (some) of the ultimate truth, but combined with an appreciation of the limits of discursive reason.
Aquinas falls short on originality. Although not a mere synthesizer, his thought derives from the Platonic-Plotinian and Aristotelian traditions. I say this as someone who appreciates his original thoughts about esse, Being. There is also the troubling and difficult question of how independent he was in his thinking: to what extent is the purely philosophical part of his work guided by antecedent faith-commitments? To what extent is it independent inquiry? Did he stray too far from Athens and too close to Jerusalem? There is no denying the tension between the two.
I would give top honors to Plato. Near the top: Aristotle, Aquinas, Descartes, Leibniz, Kant. But after Kant, it is pretty much downhill. Some would say that the rot began with Kant. Though not a conceptual relativist himself he arguably sets the stage for that 20th century aberration that reaches virulent forms in Nelson Goodman's versionism and in other forms of what Plantinga has called "creative anti-realism."
Russell doesn't come close to greatness. As Feser remarks, his writings on ethics and religion are "drivel," which may be harsh but is in the vicinity of the truth. His work in logic, epistemology, and metaphysics, though brilliant and influential, is largely derivative from Frege and Hume. On the topic of existence, I believe he is just dead wrong. Surely, a philosopher who is wrong on fundamental questions cannot be called great.
One can see that any judgment as to philosophical greatness is bound to be hotly contested.
I like Feser's untimely (in Nietzsche's sense of unzeitgemaess) remark that Nietzsche is an "ass." He has been enormously influential for more than a century now, but what is one century out of twenty five? How can one call great a philosopher whose fundamental doctrines are utterly incoherent? I am thinking of his perspectivism in particular.
Compared to earlier centuries, the 19th century was not particularly good, and the 20th was a disaster. A century in which Wittgenstein and Heidegger count as great is an impoverished one indeed. Wittgenstein, as J. N. Findlay once remarked, took every wrong turn a philosopher can take. Historically, he was an ignoramus, and his conception of philosophy miserable: the great problems of philosophy are for him nothing more than conceptual tangles that result from "the bewitchment of our understanding by language" (die Verhexung unseres Verstandes durch die Sprache). If Wittgenstein went off the shallow end, Heidegger went off the deep end: philosophy is over and to be replaced by something he called Denken, but never was able to explain to anyone's satisfaction. (Das Ende der Philosophie und die Aufgabe des Denkens.)
So perhaps I should add a seventh criterion: one cannot count as a great philosopher if one has a miserable conception of philosophy.
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