How do you know you are real?
It’s an obvious question until you try to answer it.
Meditations on First Philosophy
In his Meditations on First Philosophy
All his knowledge had come from his sensory perceptions of the world. Your senses show you the world as it is. They aren’t deceiving you but sometimes they do.
You might mistake a person far away for someone else or you’re sure you’re about to catch a flyball, and it hits the ground in front of you. But come one, right here and now, you know what’s right in front of you is real. Your eyes, your hands, your body: that’s you.
What if you are dreaming?
Dreams feel real. You can believe you’re swimming, flying or fighting off monsters with your bare hands when your real body is lying in bed.
When you are awake, you know you are awake. But when you aren’t, you don’t know you aren’t. So you can’t prove you aren’t dreaming. Maybe the body you perceive yourself to have isn’t really there. Maybe all or reality, even its abstract concepts, like time, shape, color and number are false, all just deceptions concocted by an evil genius.
Descartes asks if you can disprove the idea than an evil genius demon has tricked you into believing reality is real. Perhaps this diabolical deceiver has duped you. The world, your perceptions of it, your very body. You can’t disprove that they are all just made up.
You can’t be nothing if you think you’re something
Even if you think that something is nothing because no matter what you think, you’re a thinking thing, or as Descartes put it, ‘I think; therefore I am’
Complement Meditations on First Philosophy 
with Martin Heidegger on Being and Time.