How do we know that anything is real? This isn’t a question that usually bothers most people, because we’ve all been brought up to look upon the physical world “out there” as a given. But let’s say that someone actually asks you the question, “How do you know the physical world is real?” What would you answer?
If you pause for a second, there are only two kinds of answers to this question: Either you tell a story or you refer to your own experience. Stories used to be collective myths, generally based on religion, about how God or the gods created the world. But any story, including the most advanced scientific models, depends on belief. If you believe in the Book of Genesis, you will see reality very differently from someone who believes in the Big Bang.
To sort out which story is actually true, the second kind of answer arose, defining reality according to our experience. A rock is hard because two people who kick it agree from their experience that it is, in fact, hard.
A scientist might loudly object that we’ve left out physical data and facts. Surely a rock is hard because geologists have assembled all kinds of facts about all kinds of rocks. But such facts also come down to experience. There are rock-eating bacteria deep in the earth, and to them a rock is food, so hardness doesn’t enter their experience—for all we know, granite is to a bacterium what a lettuce leaf is to us. “Hardness” is not objective. Neutrinos are particles that travel through the Earth by the trillions and more, being impeded no more by a solid planet than if it was a puff of air, or nothing at all.
No matter how insistent you are about scientific fact, all facts enter our minds as either sensations, images, feelings, or thoughts, which UCLA psychiatry professor Daniel J. Siegel has shortened to SIFT. SIFT is necessary in order for anything to be real to a human being. Since the stories we tell and hear are also based on SIFT, especially on thoughts, it turns out that reality gets much messier than it would at first appear.
Facts, data, experiences, and stories cannot be disentangled into neat categories that everyone would agree on. Thus a specific perception of the world, such as Newton looking upon gravity as a force, got entangled with Newton’s parallel belief that the Book of Genesis was factual, and when Einstein turned gravity from a force into a matter of curved spacetime geometry, a new entanglement of facts, data, experiences, and stories came about. In Einstein’s case the Book of Genesis didn’t enter the picture, but he certainly had his own religious notions and philosophical ideas that others might or might not agree with.