In Marvel’s Dr. Strange, Benedict Cumberbatch’s titular arrogant genius has his mind blown when the Ancient One shows him the world beyond his senses. “You’re a man looking at the world through a keyhole,” she says. “What if I told you that reality is one of many?”

This concept — that “reality” is wholly dependent on our perception of it — is the central idea of the new book Deviate, written by neuroscience professor Beau Lotto of University College London. In it, Lotto takes a page from the Ancient One’s book and argues that reality has nothing to do with what’s physically in the world and everything to do with what and how we perceive the stuff that’s actually out there. When asked how this idea applies to differentiating between mental phenomena and what’s actually real during a Reddit AMA on Monday, Lotto began by succinctly defining reality in biological terms.

“[We] know what is ‘real’ according to what is useful. Evolution isn’t terribly interested in reality. It’s interested in what enables you to ‘not die’. Hence perception (and behaviour more generally) is about what helps you to survive,” he wrote.

“So what is real for us is what proved useful in the past.”

Reality is all in the mind.

Lotto’s research has led him to believe that the perceptions that kept ancient humans alive — say, their relative distance to the stars, which helped them navigate, or the warning sound of a far-off lion’s growl — may not be a true reflection of what’s really out in the world. Maybe the stars were actually much closer to them than their eyes let on. Perhaps the lion was actually much farther away than it sounded. Lotto’s argument is that — assuming we’re talking about the “reality” in which we all share — what’s actually out there is irrelevant in an evolutionary sense; all that matters is that we perceive things in a way that allows us to survive.

Later in his explanation, he cites language as a clear example of something that’s part of our reality but only exists in our collective imagination. “It doesn’t exist without us,” he writes. “But it is very much part of our reality, because it was useful for it to be so.” Scientists have theorized that the human capacity for language is what allowed us to form the social bonds we needed to survive as a species. In Sapiens, the historian Yuval Noah Harari expands on this concept of “shared fictions,” explaining that things like money, laws, and religions are likewise very much a part of our reality because they’re useful to us but also don’t exist, in any concrete and objective way, anywhere outside of our minds. If we collectively stop believing in any of these things, they stop existing — and therefore are no longer part of reality. Perception, he concludes, is all in the brain. And because we’re now learning how to manipulate perception in the brain, it’s entirely possible that we can thereby alter our experience of reality.

Lotto’s work suggests that the “keyhole” that constrains Dr. Strange’s perception of reality is the result of millions of years of natural selection working on our perceptual organs. If he’s right, however, unless we’re fighting Marvel villains in multidimensional space, what we perceive through that keyhole is all we really need to survive.