Like many other people—over $820 million worth of people in the US alone, according to the latest box office results I read while writing this post—I just saw Guardians of the Galaxy Vol. 2. It’s lots of fun—psychedelic colors, battles in space, and great ’70s music played on a Walkman.

It also features an unhappy and ever-expanding brain masquerading as a planet and seeking world domination. And that’s a problem, a very big problem, for the guardians to solve. Without giving away too much of the plot about how the guardians encounter the evil brain (it’s a pretty thin plot anyway; all the fun’s in the action), the problem for the guardians is how to counter the megalomania of the brain.

In order to understand and ultimately defeat the evil of the brain, the guardians need to figure out what’s driving it. There’s a famous philosophical thought experiment about a brain in a vat that might help them. The thought experiment, originally proposed by Hilary Putnam, is rooted in Cartesian skepticism: how do you know all your experiences aren’t illusory? One version goes like this. Imagine the possibility that you are a brain in a vat, artificially maintained and stimulated, just a brain, floating there. Wouldn’t it still be possible for you to have all the experiences, memories, thoughts, or emotions that you are now having? So, how do you know you aren’t a brain in a vat?

Putnam’s answer to this, called “semantic externalism,” was that your thoughts can’t be thoughts of something unless they refer to something that actually exists. Your thoughts might be thoughts of something else—vat-trees—but they aren’t thoughts of trees. For you to have thoughts of what we mean by trees, there must be real trees.

Now think about love. Could a brain in a vat be in love? Or would the emotions experienced by the vat-brain only be vat-brain love of a vat-brain object? (Could you love your partner if your partner were only a figment of your imagination?) Suppose the brain realized this—that the love it feels might just be the love of a vat-brain object, not a real object? That’s the skeptical point, after all—that the object the vat-brain loves really is just a vat-brain-object, and the vat-brain can’t be sure of the difference.

How might the vat-brain respond, if this recognition made it feel lonely or sad? Might it want to try to make a real object for its love? And if it realized that the object it had made was only a vat-object, not a real object, what then? How would the vat-brain react if it came to the realization that what it thought was reciprocation of its love was only vat-thinking-reciprocal-vat object? Might it get mad and terminate the vat-object for not being real? Kill off the vat-object, to try again? Might it then make another vat-object, and another, and another, each time coming to the realization that it was destined to be frustrated in love because vat-brain-love of a vat-object isn’t real love of something real?  

Even worse, think about power. Suppose the brain in a vat wanted power. Could it have power, if there wasn’t something for it to have power over? (Can you be powerful without something to lord over?) Suppose the vat-brain realized that in order for its experiences of the exercise of power to be real exercises of power, there must be external objects for it to have power over? On the skeptical charge, it can’t know whether there are such objects, so it can’t be sure whether what it thinks is the exercise of power is just illusory. What might it do then? It could try to create things to have power over, but once again the problem is that they might only be vat-things, not real things. So like the love of vat-objects, power over vat-objects might seem empty. At least, the vat-brain has no way of telling whether what it thinks is the exercise of power is simply illusory.

Confronted with these despairing conclusions, what might a vat-brain do? Make itself so big that there’s no room for anything else in this and every other galaxy? Maybe the next best thing to exercising power over external objects would be to deny their very possibility, swallowing up the whole world and in so doing extinguishing any possibility other than the pure exercise of power. 

Is this what the vat-brain thought experiment might ultimately become: the megalomania of the vat-brain? A world in which the vat-brain can be sure vat-brain is all there is, rather than a world in which the vat-brain worries about whether it can ever reach out beyond itself? Such megalomania is the frightening prospect faced by the galaxy’s guardians.

These thought-experiments aren’t just idle curiosity. They ask you to think about where radical skepticism might really lead you. Could it lead you to a kind of megalomania? If you can’t know that you are a being in the world, in real relationships with real things in the world, maybe you just are the world, the whole world, and you can do anything you want with it. Maybe that’s all there is for you—and the ultimate in the recognition that all you care about is you and the pure hegemony of you.

But if you think this way, and you are wrong and we really do exist outside of you, we just might need some guardians to protect ourselves against you….