Descartes makes methodological solipsism almost compulsive for later philosophers. (How is that for opening with a grand pronouncement?)
By “methodological solipsism” I mean an approach to philosophy that claims we have privileged access to our own mental states, we have such access to nothing else, and one of philosophy’s primary tasks is to show how to get from subjective states to whatever else we wish to accept or justify. Descartes thought he could make the jump from subjective “ideas” to external “substances” by proving that there is an all-good and all-powerful God, who would not allow us to be deceived by our “clear and distinct ideas.” Philosophers in the empiricist tradition, like Hume, tried to show how our knowledge of the world could be constructed out of our “impressions.” Most philosophers thinks both failed in their respective projects.
Methodological solipsism is a seductive line of thought, and is hard to escape once it ensnares you. But why should we accept it? There are at least two ways to argue for it.
(1) Descriptively: we do, in fact, “start” from subjective experiences. This seems obviously false, though. Is the claim that, when I wake up in the morning, I have experiences that are properly categorized as purely subjective, and that I go through a process of gradually inferring the existence of other things? I don’t do that, and I doubt if others do. I wake up to what I experience as a world of independent objects, of which I am a part: alarm clock, bed, wife, my body, etc. Experiences that we regard as subjective are very special cases. Perhaps I hear a “ringing” in my ears, and I wonder whether it is a real sound, or just a symptom of an ear infection. I ask other people whether they hear it, and if they do not I take an aspirin and the ringing goes away. But certainly looking at my dog and hearing him bark is not like that.
(2) Prescriptively: although we normally experience what we take to be a world of independent entities of which we are a part, all we are entitled to accept as starting points of warranted reasoning are our subjective experiences. But what is inferentially special about our subjective experiences? Descartes said that we cannot be deceived about the fact that we are thinking. But I certainly can be deceived about the content of my ideas. Descartes likes to talk about dreams. The content of dreams is notoriously inchoate, though. Did I dream that I held a cat? Or was it a skunk?
“So much the worse!” crows the skeptic. “You see that you cannot even be certain of the content of your subjective experiences.” True, but this just raises again the issue of why the methodological solipsist insists that we privilege them. Our subjective experiences are one particular kind of our experiences, and they are often very difficult to single out and identify. I’m looking at a computer screen now. It requires a lot of mental discipline to focus on my subjective experience of the screen, rather than the screen itself. Why not take for granted my belief that I am looking at a computer screen, unless a specific concern leads me to doubt this particular belief.
So why not conduct philosophy the way we conduct ordinary conversations? We are fellow humans who share a common world. We sometimes discover that what we see in the world is different. And that leads us to talk about why we see the world that way. Philosophy is that conversation about what and why we see in the world.
Of course, you can disagree with me, but if you’re a methodological solipsist you’re just disagreeing with a disagreeable subjective experience. Just take an aspirin and maybe I’ll go away.