When I was in college, the daily student newspaper included a really challenging crossword puzzle that I enjoyed working on every day. That is to say: I didn't pay attention in class most of the time. Very few lessons have stuck with me over the years. But one class I really enjoyed was Philosophy 101, where we talked about ideas like Plato's Allegory of the Cave.
In it, the Classical Greek thinker imagines prisoners who have spent their entire lives chained in a dark cave, unable to move their heads, forced to watch shadows from a fire move across a dark wall. All they know of the world--of existence in any form--are these shadows.
The metaphor is an explanation of Plato's Theory of Forms--that appearance is not necessarily reality. A lot of contemporary works have channeled this idea, such as The Matrix. (Brilliant though he was, Plato could not have conceived of a Keanu Beeves.) And some people, Tesla's Elon Musk among them, even believe that we really exist only in a futuristic computer simulation. There's no way to prove or disprove it.
But I tend to combine Plato's idea with another from the 17th-century French philosopher Rene Descartes, who said cogito ergo sum: I think, therefore I am. In other words, because I can think and hear my own internal dialogue, I have proof of my own existence. Combined with Plato's allegory, though, I cannot necessarily prove the existence of anyone or anything else. I cannot prove that my earthly body or my girlfriend or my car actually exist. They are the equivalent of the shadows on the wall of my cave. For all I can definitively say, I am the only cosmic mind that there is.
That's a pretty mind-boggling, complicated thought, and an extremely lonely one ... unless you combine it with the philosophy of Immanuel Kant.
Kant devised the "categorical imperative." As he put this law of man: "Act only...