For philosophers in many traditions, dreams and meanings– or definitions– of reality have been inextricably linked. Dreams have provided human beings with worlds and experiences that can be just as vivid as real life and yet involve places, objects and people that do not exist in our waking lives. Vivid dreams can be seem totally real as we’re having them, a realism that does not diminish if you become lucid during the dream: the dream’s reality may actually increase as you consciously engage your dream senses with the environment. However, lucid dreamers are also aware that these realistic scenarios and environments are a creation of their minds. At some point, people who have vivid or lucid dreams may question whether the reality they perceive through their waking senses is, in fact, any more valid than a dream. The convincing quality of dreams has led philosophers from Zhuangzi to René Descartes to ask the big question, “Can we be certain of anything?”

The “dream argument” postulates that the act of dreaming provides evidence that the senses we trust to distinguish reality from illusion are untrustworthy, and that we should rigorously test and question any reality that depends on information we receive from our senses. In fact, we should perform the same kind of reality checks we would make in dreams. However, Descartes believed reality checks might not be enough to determine that you are not dreaming at this very moment. “Wait,” you might say, “Dreams don’t hang together like real life. Impossible things happen in them. Even the most coherent dream can have wonky gravity or physics that lets you know it’s a dream.” But what if you are just in the middle of a very coherent dream, one your brain believes in so completely that reality checks don’t work? False awakenings are a perfect example of a situation where you believe you’ve woken up and may not think to check reality at all— or if you do, the dream reality defies your expectations by remaining consistent. Descartes used these kinds of arguments to undermine people’s confidence that the world around them was unquestionably real.

He also used a thought experiment involving an evil demon to illustrate this point. Suppose that there is a demon powerful (and mischievous) enough to present a complete yet illusory world to your senses, including the internal sensations of your body. In this scenario, you would be completely unable to tell that the construct the demon is presenting you with is unreal… which means you would have no way of telling what the world beyond it is like, or if there is even a real world beyond it at all. Even what you think of as your body would be part of the illusion. Going back to dreams, you could think of your brain in the role of this demon, presenting you with a completely real-seeming dream world as you sleep, constructed using mental approximations of the same senses you use to process information while awake. This means that you cannot rely only on your senses for verification that the waking world is real. What Descartes concluded was that the only thing you can be absolutely sure of is your own existence, summed up in his famous maxim, “I think, therefore I am.” You can believe that the real world is an illusion and that you don’t have a body, but you cannot believe that you don’t exist in some form; otherwise who would be thinking those thoughts? In other words, you can believe that your entire life is a dream, but something called you must still be around to have the dream.

However, there are problems even with this view, one of which is expressed by the Dzogchen school of Buddhism. Dzogchen proposes that everything we sense with our external thoughts is an illusion or dream, and this extends to thoughts based on external sensory information. For most of us, a majority of our thoughts and feelings derive from our external experiences in what we believe to be “the real world”, but to a Dzogchen Buddhist, externally-based thoughts are complicit in perpetuating an illusory reality. The Dzogchen school holds that meditation can help the practitioner quiet these misleading thoughts and get in touch with the Self of pure awareness that exists independent of our thoughts.

The question “Can we be certain of anything?” has also led to three main streams of thought in Western philosophy: materialist, objectivist, and idealist. Idealists believe that if our reality is generated by our perception, then perhaps people’s different perceptions create different realities. In this framework, there is no single real world at all; rather, each person lives in a different equally valid reality. The opposite approach, materialism, holds that there is just one reality that exists regardless of people’s perceptions and beliefs. Scientists ascribe to methodological materialism (or naturalism)  to generate hypotheses and design experiments. Finally, objectivists tread the middle way by agreeing that people may perceive reality differently, but different perceptions do not create or alter what that reality is.

Many of us are probably objectivists: there are practical reasons for agreeing that reality is a certain way. Most people ascribe (at least in their everyday lives) to consensus reality: defined as what is real, or what seems to be real, to the greatest number of people. Consensus reality offers us a practical guide for navigating the world and making decisions based on the information we get from our senses. In dreams, we operate within a consensus reality of one: if you subscribe to Descartes’ skeptical approach, reality checks within dreams can prove, at most, that you’ve entered a different reality than the consensus reality you inhabit while awake. Whether that world is any more real than your dream will rest on your beliefs.