“Being one with the universe” is perhaps one of the most joked about parts of spiritual practice among people who aspire to be logical and scientific, such as myself. However, I think that this phrase can be explained in a fully secular way that makes a whole lot of sense while being extremely less poetic.
First off, as I describe in my beginner’s guide to rationalist meditation, you should see most writing on meditation as not describing the world as it is (despite that usually being the intentions of the writer) but as describing how the experience feels. So most of the time, if somebody says “the cosmos is consciousness”, the way you can understand that is to add “it feels like” to the beginning of the sentence, so turning it into “it feels like the cosmos is consciousness”.
In the case of “being one with the universe”, it is the feeling that you and your body are not separate from the universe, but a part of it. Many have had this experience with hallucinogens, where when you look at your hands or body, you don’t have the usual feeling of them being distinct from the rest of your visual field, but it is all one big unit.
Not only is the feeling quite enjoyable, it also is, in this one case, in a certain sense true metaphysically. As a hardcore determinist and materialist, I do not believe in souls or any sort of supernatural things that exist outside of the laws of the universe. Everything is explainable, and if we don’t understand it, there’s always a “yet” at the end of that sentence. We don’t understand it yet, and we just have to keep trying to figure it out. And right now, the best available explanation is that we’re all matter and energy, and that the matter and energy inside your body is not fundamentally different from the matter and energy in the floor, your microwave, or the stars. We feel like there’s some sort of separation or distinctive factor about us, hence the common myth of souls or “essences”, but these are mental illusions or delusions. With enough targeted meditation practice, you can actually see this illusion. You can see that you are atoms flowing among other atoms that are not separate from you in the sense that your blood cells are not separate from your body, but just a component. This is part of what people are gesturing towards when they say that they are “one with the universe”.
I have had flashes of this experience while meditating and outside of it (it’s a very strange experience to have while talking to somebody!) and I’m learning to stabilize and bring it up on demand. It is highly enjoyable and leads to a sense of elation and calm at the same time. I would highly recommend it and all the other meditative states I have experienced so far. If you’re interested in learning more, I’ve listed some other pieces I’ve written about meditation below where I try to explain it as clearly and logically as possible without getting into any woo.
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I'm an effective altruist who co-founded Charity Science Health and Charity Entrepreneurship.