But I have convinced myself that there is nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I too do not exist? No: if I convinced myself of something, then I certainly existed. But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case I too undoubtedly exist, if he is deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something. So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind. [From Meditations on First Philosophy]

Ah, yes: the cogito, in all its glory. So, I'm basically saying that I definitely exist. But what am I actually talking about? It's funny you should ask…

First, the backstory. This was all part of my quest to find a solid foundation on which I could build a stable and enduring system of knowledge. Is there any belief which is absolutely certain and indubitable? That's what I wanted to find out. With that goal in mind, I set out to reject as false any claim that was at all capable of being doubted.

Now, we're not talking court-of-law reasonable doubt here—we're talking tiniest possible shred of logical doubt. I go so far as to envision a scenario in which an all-powerful demon is deliberately concerned to deceive me as far as he possibly can. Is there anything I can still know? Can I be certain there is a world out there—is there actually a table in front of me? No—this could all be a hallucination caused by the evil demon.

But it's not only the world out there that's doubtful; my own physical body might be nothing more than a demon-caused illusion, too. That's beginning to worry me, though I guess that might mean I could skip the gym on Tuesdays. The scary thing is that this demon could be misleading me about even simpler, more basic things—making me falsely believe that 2+2=4, for example, or that a square has four sides. Doesn't everything become doubtful?

No. Because even if I am being misled about everything, I know that I am being misled. Even if I doubt the existence of everything, I know that I am doubting. Let the evil demon do his worst, but from the very fact that I am thinking, doubting, being misled, it necessarily follows that I exist!

The indubitable truth has been found.

But even as I speak, I put the wax by the fire and look: the shape is lost, the size increases; it becomes liquid and hot; you can hardly touch it, and if you strike it, it no longer makes a sound. But does the same wax remain? It must be admitted that it does; no one denies it, no one thinks otherwise. So what was it in the wax that I understood with such distinctness? Evidently none of the features which I arrive at by means of the senses; for whatever came under taste, smell, sight, touch or hearing has now altered—yet the wax remains…I must therefore conclude that the nature of this piece of wax is in no way revealed by my imagination, but is perceived by the mind alone. [From Meditations on First Philosophy]

Here, I am describing my famous "wax experiment." It's all about me trying to answer the question of what the identity of an object consists in, and how we make that determination. You know you've lain awake many nights worried about that same question—admit it.

So, consider this piece of wax, first before it is brought before the fire (BF) and then after it is brought before the fire (AF). BF, the wax has a number of definite qualities—a certain size, shape, smell, and so forth. All these qualities are reported to me by my senses. But AF, all these qualities have changed. Yet it is still the same piece of wax, is it not?

I thereby show that the senses alone do not tell me anything about the real nature of the wax. My senses report that BF and AF are totally different, and yet I know that the object in front of me is the same. How do I know? Because it's the mind that makes this determination.

Think of it this way: I might say that I see a man walking down the street, but what I actually see is just a coat and maybe a hat. It is my mind that judges or infers that this is a man rather than, say, a body snatcher. I'm not saying the mind always judges correctly, of course—my view is that body snatchers are way more common than most people think. Call me paranoid, if you like, but I have a pretty good track record of being right in my paranoid musings.

My grand conclusion is that it is the mind and the mind alone that tells us what the world is really like. This is the basis of my whole view of "rationalism."

So there remains only the idea of God; and I must consider whether there is anything in the idea which could not have originated in myself. By the word 'God' I understand a substance that is infinite, independent, supremely intelligent, supremely powerful, and which created both myself and everything else (if anything else there be) that exists. All these attributes are such that, the more carefully I concentrate on them, the less possible that they could have originated from me alone. So from what has been said it must be concluded that God necessarily exists. [From Meditations on First Philosophy]

This is my main argument for the existence of God. You will appreciate my ingenuity even more if you realize what my starting point is: according to my argument at this juncture, all I know for certain is that I exist. How do I know there is anything else—bodies, other minds, fellow Shmoopers? All of this has to be proved, somehow, without my assuming the existence of anything other than my own mind.

Well, my first step is to prove God's existence. Pretty natural, right? Obviously, before I can know that the table in front of me exists, I first have to know that a supreme being exists. (If that doesn't make sense, read on.) So, what I do is I show that the idea of God is the one idea in my mind that I could not possibly have caused.

Now, I need to make a few tiny little assumptions to make this argument work. The main assumption is a little something I call Descartes's Principle (it has a nice ring to it). You remember the distinction I drew between formal and objective reality? That comes into play here. Descartes's Principle states that there must be at least as much formal reality in the cause of an idea as there is objective reality in the idea itself.

Sounds pretty impressive, I know, but it's very simple and actually quite obvious once you think about it. All I'm saying is that whatever causes an idea must be at least as real—as perfect or complex—as what that idea is about. If I have an idea of a purple cat, the cause of this idea has to be at least as perfect or complex as a purple cat.

As I say, it's pretty obvious when you think about it. Suppose there is some guy named Jack who you and everyone else thinks is a total doofus. But then it turns out Jack has just come up with an amazing, complex invention. And so you say, "Oh, well—I was wrong about old Jack. I guess he is actually a total genius." That's Descartes's Principle in action: we assume that the mind that comes up with an idea has to be at least as perfect or complex or evolved as the idea that it produces.

So I go through some of the ideas in my head—ideas of rocks, animals, Desiree Hartsock. There is nothing in those ideas that is so complex that, according to Descartes's Principle, my own mind could not have created them. (Now I admit that I would not have thought that I could have come up with a scenario where someone accepts a marriage proposal from a guy after telling some other guy just two days earlier on national television that he was the true love of her life. But, then again, maybe I'm just more imaginative than I realized.)

But then I come to my idea of God. (This is just where the above quote comes in.) According to Descartes's Principle, I could not have caused this idea myself, since the idea is of an infinite being, while I am only a finite being; the idea is beyond me in complexity and perfection. Instead, by my principle, only an actually infinite being could have caused this idea. Therefore, God must exist, as the cause of my idea of God.

Now, I suspect this proof won't quite make you ready to accept God's existence if you are not already so inclined. But you have to admit it's pretty clever. (Somehow that seems to be the effect of most of my arguments.)

Now there is in me a passive faculty of sensory perception, that is a faculty for receiving and recognizing the ideas of sensible objects; but I could not make use of it unless there was also an active faculty, either in me or in something else, which produced or brought about these ideas. But this faculty cannot be in me, since clearly it presupposes no intellectual act on my part, and the ideas in question are produced without my cooperation and even against my will. So the only alternative is that it is in another substance distinct from me—a substance which contains either formally or eminently all the reality which exists objectively in the ideas produced by this faculty (as I have just noted). This substance is either a body, that is, a corporeal nature, in which case it will contain formally everything which is to be found objectively in the ideas; or else it is God, or some creature more noble than a body…[But] God has given me no faculty at all for recognizing any such source for receiving these ideas; on the contrary, he has given me a great propensity to believe they are produced by corporeal things. So I do not see how God could be understood to be anything but a deceiver if the ideas were transmitted from a source other than corporeal things. It follows that corporeal things exist. [From Meditations on First Philosophy]

Okay, I'll admit that this is one of those rare cases where I was just a tad long-winded. For the benefit of all you Shmoopers, I will restate the argument much more briefly. I'm proving the existence of the so-called "external world," a world of physical objects. You got that much, right?

Up to this point, all I have proven is the existence of myself and of God. How can I prove there is anything else out there? Things, for example. How do I know they exist? I have lots of ideas of physical things—for example, I've got an idea of a table in front of me right now. But how do I know that there is anything that corresponds to that idea? How do I know it's not just all in my mind?

Well, everyone told my Aunt Agnes that her many complaints about her health were all in her mind, and then she dropped dead one day. See? You can only go so far with that kind of claim. Anyway, that was my original argument for the existence of the external world, but a lot people seemed unconvinced. So I came up with something still better.

In a nutshell, I decided I had to figure out what was causing my ideas of physical objects. One possibility: some faculty in my mind that I don't know about is causing them. But, nah—it can't be that, since these ideas sometimes arise against my will. This pain I experienced after smashing my thumb with that hammer? Definitely not my choice.

Then there are only two other options. Either God is causing me to have these ideas, or actual physical things are causing them. I'll admit that there is no way I can tell the difference here: things would appear exactly the same in either case. But I definitely have a strong tendency to assume that physical things themselves are causing my ideas of physical things. And if God set things up in such a way that this belief was mistaken and yet he gave me no way to determine what the truth really was, he would be a great deceiver.

Think about it: could God sit back and say "Haha, got you, sucker!" as he saw me over and over again falsely claiming that an external world exists? Never. The God whose existence I proved is benevolent. Therefore I know it's not God but actual physical objects that are causing my ideas of physical objects. And that means that a world of physical objects exists independently of me.

Now, these objects may not, in themselves, be exactly as they appear to me. But at least I can be certain that they exist. So, rejoice everyone—your iPhone is not just a figment of your—or God's—imagination.