Comrades:

Benedict Spinoza

I can't say I ever met the guy, nor can I say I agree with everything he wrote—not by a long shot. This dude rejects my central idea that there are two kinds of substances, minds and bodies, and instead insists that there is only one substance? One substance? Ooookay.

Still, the basic approach to philosophy—he gets it, he really gets it. Like yours truly, Spinoza is a rationalist. He accepts rationalism's key principle, which is that we can arrive at the fundamental nature of reality through the use of reason alone.

For us rationalists, mathematics is the paradigm for all genuine knowledge. In my "Rules for the Direction of the Mind," I go so far as to suggest the possibility of a whole philosophy modeled on mathematics. I thought that was pretty good, but my buddy Spinoza really ran all the way with the idea. He wrote a book called the Ethics, and it's literally modeled on Euclid's geometry, complete with definitions, axioms, propositions, and scholia. It is, of course, a bit mad, but it is clearly Cartesian-inspired madness, and so I approve.

Gottfried Wilhelm Leibniz

This guy rejects my idea that there are causal interactions between minds and bodies and comes up with some notion of "pre-established harmony" instead. According to this idea, each substance only interacts with itself, but God has harmonized everything in such a way that substances appear to be affecting each other.

Sounds pretty crazy, right? My finger would be drawing clock-wise circles around my ear right now, if that weren't unbefitting someone of my stature.

But despite that zaniness, Leibniz, just like Spinoza and me, has the right fundamental approach: he's a rationalist, through and through. He, too, envisions a mathematized version of philosophy—he even wants a special language (he calls it characteristica universalis) to go along with it. Not bad.

Henricus Regius

There are lots of people who have followed my ideas in physics and physiology—too many to name, really. And lots of people followed me in philosophy, as well. (Or tried to, anyhow.)

Regius I would have to put in the latter category, as someone who made a real effort. He was a good man, and he was very enthusiastic about my ideas. But he could never quite get the point. (Where did he ever get the idea that the mind is a "mode" of the body?) Finally, I had to set him straight by publishing several letters in which I detailed all of his errors.

Thomas Nagel

My psychic friends tell me that in the 21st century, my ideas will have fallen out of fashion. Apparently, mind-body dualism is considered passé, and almost everyone is instead some form of physicalist. But then there's Nagel.

Now, of course, Nagel won't use my language of "mental substance" and "corporeal substance." No, he likes to talk about "consciousness" and "the physical world." (Everyone has to try to be so original.) But his basic conception of a private inner world absolutely distinct from an outer material domain—that's pure Descartes. I love it.

Rivals:

Aristotle

To be fair, my real rival is not exactly Aristotle, but Aristotle as he was interpreted by the medieval Scholastics. So let's call my enemy the Scholastic Aristotelians. What did I disagree with them about? Can I just say everything? Okay, how about I say everything important.

If I had to name just one thing, though, the key difference between the Scholastics and me has to do with the their belief in Aristotle's notion of final cause. This is the idea that you can only really explain something after you've specified its ultimate goal or purpose. But no one who understands the new sciences would say such a thing. Nature has no purpose.

So, I rejected the Scholastics' approach in favor of one that emphasized only mechanical explanations. For me, nature is like a machine, and change and motion are to be explained simply as the result of external forces. Any other sort of explanation has nothing to do with physics; it's just hocus-pocus.

John Locke

Locke has the nerve to challenge my claim about the existence of innate ideas. I suggest, quite reasonably, that the basic truths of mathematics, logic, and metaphysics are inherent within the mind and are known through clear and distinct cognition. But Locke says there are no such ideas. He actually believes that the mind at birth is—get this—a tabula rasa, or blank slate, and that all knowledge is gained through experience.

Sounds like a bit of nutcase, wouldn't you say?

Thomas Hobbes

Hobbes is a brave man—I'll give him that. He had the guts to actually send me his criticisms of my views rather than just talk about me behind my back, like most of my enemies. He makes (or tries to make) a number of objections to my Meditations, but the key one has to do with the cogito. He claims that I have not shown the mind to be immaterial—he thinks that it could actually be a physical thing (source).

I demolish this bit of silliness in a printed reply.