Let us begin with Descartes, since arguably it is with him that the present story has its origin. In his Meditations on First Philosophy (1641), the great French philosopher and mathematician lays out a thought experiment meant to undermine the supposed foundations of his commonsense beliefs. Let me imagine, Descartes writes, that a powerful deceiver is the source of all my thoughts and perceptions. When it seems that I am opening my eyes and seeing the stove in front of me, my vision is the product of a cunning deception. When it seems that I am raising my arm to scratch an itch, my feeling of embodiment is another of the deceiver’s illusions. And so, too, for my sequences of logical and mathematical reasoning: these are the deceiver’s art. In the face of this hypothesis, Descartes asks, what could permit me to rely on my senses or my reason as ways of knowing how things are?
Along with this skeptical thought experiment, Descartes’s most famous contribution to the history of philosophy is the first argument he offers in response to it, which identifies one’s own existence as the first thing a person is able to know with perfect certainty. I think, therefore I am was Descartes’s formulation of the crucial argument in his Discourse on Method (1637)—and from this it follows that what I fundamentally am is not a human being, nor anything that takes up bodily space. In knowing himself, Descartes knows himself to be a thinking thing, and he knows this prior to his knowledge of anything that is bodily or “extended,” such as his eyes, his arms, and the furniture in the room around him. Indeed, Descartes reasons, I myself could exist as the thinking thing I am, with just the thoughts and perceptions that I know myself to have, even if everything “external” to me and my thinking were imaginary.
In contemporary philosophy, the most compelling reconstruction of Descartes’s skeptical scenario is in a 1981 article by Hilary Putnam, which explores the cognitive situation of a “brain in a vat.” In Putnam’s thought experiment, a human-like brain is sustained by fluid and connected to a computer that feeds it stimulation corresponding to what our brains ordinarily receive from our sensory systems, in a way that is responsive to outgoing signals from the motor system in the brain. This image avoids what is commonly taken to be the central embarrassment of Cartesianism—namely, that Descartes’s “thinking thing” is an immaterial substance that exists outside the influence of physical laws. At the same time, Putnam’s scenario renders in even more literal terms the idea that thinking, perceiving, and acting are internal processes that must be connected somehow to objects in a realm outside them. Descartes’s immaterial soul may have been a metaphysical extravagance, but Putnam’s brain in a vat provides a more scientific-seeming version of the same basic picture—the picture of our mental lives as confined to a special realm. For Descartes, that realm was the domain of the immaterial soul. For Putnam, it was that of the vat.
Descartes’s ideas marked a radical departure from the understanding of human nature that had been laid out by Aristotle and developed by his scholastic commentators. But the radicalness of these ideas is likely to be lost on us, as they have captured the popular and scientific imagination no less than the imagination of professional philosophers and their students. The Cartesian picture of human nature, where the mind is something inner with a problematic connection to things in the “external world,” including our very bodies, is for many people the default way of understanding who and what we are. Putnam’s thought experiment may seem implausible, but don’t most of us already think of ourselves as essentially brains in vats? Only the vats are our human heads, the cables leading in and out are our sensory and motor nerves, and what lies outside is neither a deceiver nor a computer but the “external world” itself, doing just the same work of stimulating our brains that a deceiver, or a computer, or a sufficiently realistic and immersive virtual-reality system could in principle do instead. So it is that many of us fail to see that this picture is only a picture, and that there may be alternative pictures that provide a superior understanding of what we are.
Reality+, the latest book from the philosopher David Chalmers, is an engaging and far-reaching exploration of this Cartesian picture, by way of the hypothesis that our mental lives are the product of a massively complex computer simulation. Its central thesis is that virtual worlds are real worlds, so that even if we are living in a computer-generated simulation, or are ourselves the products of such a simulation, this is no threat at all to our own reality, to the reality of the things we take to be around us, to the truth of most of our beliefs about those things, or to the value and meaningfulness of our lives. The origin of this argument is in Chalmers’s paper “The Matrix as Metaphysics” (2003), which used that famous science-fiction film to mount an updated version of Putnam’s “Brain in a Vat” thought experiment. Most of us watch The Matrix, with its rows of human bodies hooked up to a machine feeding them images of a world nothing like the one around them, and judge this simulated world to be an unreal illusion that its subjects should want to escape. In this reaction we are in agreement with Plato, whose Republic imagines a world of prisoners chained in place and observing a realm of shadows. Anyone who was freed from this bondage and escaped to see things in the light of the sun would be obligated to return to the cave and help bring others to enlightenment. If Chalmers is right, however, this reaction would be misplaced. The shadow world is no less real or meaningful than the world beyond, and there is no deception at all in being confined to it.