Philosophy is the science whereby we find understanding through reason alone. Theologians by definitions are not philosophers, since theology is the science whereby we base our understanding on the data of divine revelation. Granted, theologeans can be philosophers also, insofar as they derive certain truths from reason alone, but insofar as the theologeans do theology, the theology that they are doing is not
St. Anselm, St. Thomas Aquinas, and St. Augustine are all rightly called philosophers.
St. Anselm does not depend on the Bible to prove the existence of God (whether or not you agree with the Ontological Argument), but rather deduces it from the meaning of the words "greater than."
St. Augustine was a Neo-Platonist (with definite Stoic influences), and St. Thomas Aquinas heavily relied on Aristotle.
St. Faustina, on the other hand, can by no means be considered a philosopher. Even though there's a lot of good stuff in St. Faustina's writings, and even though she most likely is entirely right in what she says, reason alone did not led her to writing. Jesus Christ, appearing miraculously to her, led her to writing.
Philosophy is not a what. It's not merely "truth." You can disagree with a philosopher and still call him a philosopher.
Philosophy is a "how."
Wittgenstein boy: the reason I ranked St. Anselm so highly is because of his Ontological Argument. It's one of the most famous proofs of God's existence (whether or not you agree with it) in the entire History of Philosophy. Almost (if not every) every
student of philosophy is familiar with St. Anselm, if only because of that proof.
The same is not true of Wittgenstein. Even if we agree that Wittgenstein was a good philosopher (I don't), it can scarcely be denied that Wittgenstein, even if influential, certainly is not
more influential than the other 20 that I listed.
I ranked Mill so highly because Utilitarianism is probably one of the biggest (even if wrong) philosophical worldviews out there today.
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Also, I ranked the Mideivals so highly, in large part, because they showed up on the scene earlier, and therefore much of the work that they did was, in a very real sense, seminal