The most defensible working definition is that the real is what has causal power and does not depend on any particular mind's believing in it—what would still make a difference to the world whether or not you, or anyone, took it to exist.
"Real" is not one thing, so a single master test misleads.
Two criteria do most of the work and usually coincide. Mind-independence: a rock persists unobserved, while a hallucinated dagger vanishes when the belief does, which is why we call the first real and the second not.
Causal efficacy: the real is what pushes back, what enters into cause and effect regardless of our wishes—you can test it, be surprised by it, be injured by it.
What resists the will and produces effects independent of belief is the paradigm of the real.
This admits degrees and kinds rather than a binary.
Electrons and tectonic plates are real though unseen, because they have measurable effects.
Money, laws, and nations are socially real: they depend on collective belief, yet within that scope they have hard causal power—a court can jail you, a currency can collapse—so they are real relative to the practices that sustain them, not illusions.
Pains and experiences are real as occurrences even when their contents misrepresent the world.
The error is to demand that everything be real in the same way as a rock, or to dismiss as unreal whatever isn't.
So ask not "is it real?" but "real in what sense—mind-independent, socially instituted, or experiential—and with what causal reach?" Treat as real whatever you must take into account to act successfully and would be surprised or corrected by if you ignored it.
This picture would need revision only if some purely mind-dependent thing with no causal effects nonetheless demanded to be called real, which nothing clearly does.
The most useful definition: something is real if it has causal power and doesn't depend on your believing in it—if it would still be there, still making a difference, whether or not anyone thought it existed. Reality is what pushes back.
Two tests do most of the work, and they usually agree:
The trap is thinking "real" is all-or-nothing, like everything has to be real the way a rock is. It isn't:
So stop asking "is it real?" and ask "real in what way, and with how much causal reach?" Practically: treat as real whatever you'd get punished by reality for ignoring. If ignoring it gets you surprised, corrected, or hurt—it's real enough to plan around.
That Which Won't Go Away.
Philip K. Dick offered the sturdiest definition anyone has managed: reality is that which, when you stop believing in it, doesn't go away. The real is what refuses you—the wall you walk into regardless of conviction, the diagnosis that doesn't care about your denial, the debt that comes due whether or not you acknowledge it. Everything else, however vivid, is on some level yours; the real is precisely the part that isn't. To be real is to be able to say no to a mind. Reality is the resistance the world offers to the wish—the friction that tells you you have run out of self and struck world.
The Realest Things Have No Atoms.
And yet the forces that most powerfully shape your life are made of nothing you can touch. Money is ink and belief, yet it builds cities and starts wars; a nation is a story millions agree to tell, yet people die for its borders; the law is words, and the words can lock a body in a cell for decades. These are "fictions," but try treating them as unreal and reality will correct you sharply. Perhaps the lesson is that solidity is a poor guide to power—that the heaviest realities in a human life are woven from shared belief, and that a thing can be entirely invented and utterly binding at once.
The Dream That Cannot Prove Itself.
Descartes lay awake wondering how he could know he wasn't dreaming; Zhuangzi woke from being a butterfly unsure whether he was now a man who had dreamt a butterfly, or a butterfly dreaming a man. The unnerving discovery is that no experience carries a stamp of its own authenticity—every dream, from the inside, feels like waking, and immersion cannot certify itself. This is the permanent crack in the floor of certainty: whatever test you run for reality, you run it within your experience, using faculties the dream would also supply. We are sealed inside the show, holding a ticket that might be forged, unable to step outside and check.
Maps All the Way Down.
Even setting dreams aside, you never touch the world raw. Light becomes signal becomes model; you experience not the rose but your brain's rendering of it, useful and edited and slightly late. Kant drew the hard line: the thing-in-itself, whatever reality is apart from our way of receiving it, is permanently sealed off, and we live entirely among appearances—maps we mistake for terrain because we have never once held the terrain. Science extends the maps magnificently, but each new instrument delivers another representation, not the naked real. It may be that "what is real" beneath all our models is a question shaped like a door with no room behind it—territory we can approach forever and never arrive at.
What You Must Answer To.
So drop the demand for a final, mind-independent bedrock you can never reach, and keep the version you can use: the real is whatever you must answer to, the part of things that does not bend to the wish. You did not author the death of someone you loved; you cannot believe your way out of hunger; the beloved's inner life is genuinely other and not your invention. And there is a strange comfort buried in this. A world that answered instantly to every wish would be a kind of solitary confinement, a hall of mirrors with only you in it. The reality of what resists you is also the reality of everything you did not make—other people, the given world, the sheer stubborn otherness that means you are not alone in here. The real is the name for that: everything that is not merely you.
The most workable meaning of "real" is what has causal power independent of any particular mind's believing in it—what pushes back, surprises, and corrects you whether or not you accept it. Reality is not one uniform thing but comes in kinds and degrees: physical things unseen but effective, socially instituted things like money and law that depend on collective belief yet bind with real force, and experiences that are real as events even when their content misleads. So the productive question is not "is it real?" but "real in what way, and with how much causal reach?" What unifies every sense is that the real is the part of the world you must answer to, the part that does not simply bend to the wish.