Why would someone believe the earth, the stars, the galaxies, space itself has one soul, and all our souls are part of this one singular soul? That's everything; rocks, mountains, trees, animals, oceans, Mars all being one.
In Latin, the word Anima mundi, means the soul of the world, our world as a living thing, with a soul and intelligence of its own. One single visible living entity containing all other living entities on the planet, which by nature are all connected.
Connected in the same way our soul is connected to our bodies. Plato, a student of Socrates came up with the idea and the theory and as far as the rest of the people at the time were concerned, pretty believable. Carl Jung even related Anima mundi to his Theory of Synchronicity. Two or more unrelated events happening together in a manner which seems deliberate, the outcome meaningful, all being pieces of the whole.
In Hellenistic philosophy all people are manifestations of the one universal spirit. In Hindi a Sanskrit word meaning the soul, the self is connected to all things. The force of nature, believed by Yin-Yang'ers, all interconnected.
Are we just a small part of a whole?
A cog in the wheel, a grain of sand, one of trillions that make up a beach. It's worth looking into and debating.