Unity and Oneness on a cosmic scale is a popular theme in dub culture. The Jamaican mystical philosopher Bob Marley made it his subject, and of course that helped. A lot. Marley also talked about Ethiopian kings, Jamaican politics and certain herbs, but One Love was his number 1 remedy for Babylonian brainwash education.
The philosophical roots of cosmic oneness are very old. The Neoplatonist philosopher Plotinus (204 - 270 AD) defined The One as the source of the world, beyond all attributes (including being and non-being). But he was hardly the first Greek philosopher to think about Monism. Heraclitus, for example, said: "God is day and night, winter and summer, war and peace, surfeit and hunger."
Many other traditions like Hinduism, Stoicism and Pantheism also embraced The One as ultimate source of reality. And even a few modern Western philosophers, including Spinoza and Hegel, thought that was a good idea.
Separation for dummies
But Babylonian brainwash education remains strong. The standard Western view of the cosmos is that it's essentially a cold, hard, meaningless place, ruled by cold, hard, meaningless laws of physics. From this point of view, reality sucks and life is a fluke. And human thought quickly becomes absurd. According to Mr. René Descartes, matter can't think, but it exists anyway. Or something like that.
Mr. Descartes famously believed that his mind was completely separated from his corporeal body. There was a mind called Mr. Descartes and there was a body called Mr. Descartes. But there was no connection between the two. Or so the Descartesses thought. Or to be precise: that's what Descartes the mind thought. The flesh and bones Descartes was too stupid to think. A textbook example of 'pure' unembodied rational thought producing irrational ism & schism.
Dualism and other kinds of separation-based thinking have affected the Western Mind for ages. It's roots can be traced back to ancient Greece. Primarily to the simplistic, overrated either/or logic of Aristotle. The problem with formal modes of thought like mathematics and them-or-us-logic is that they're ultimately meaningless. Ergo: if you only accept 'pure' formal thought, you more or less assume the Universe is directed by amoral blind fate.
Western thought is mostly analytical. But analysis without synthesis can never really answer big questions about "the meaning of life". Look at genetics for example. We analyze the DNA of life, and start tinkering with it. But we miss the hidden keys, the bigger picture.
Oneness means connectedness on every level, everywhere you look. Crucially, this implies that we have a certain amount of moral responsibility for our thoughts and for our actions. But unfortunately, to certain people, that idea is even more depressing than the concept of being ruled by cold, hard, meaningless laws of physics.