what it means to live in a holographic cosmos
← emphasis mine.
Duality is a connection between two things where the properties of one defines the properties of the other. For example, imagine a book lying on a table, with a nail taped to its front cover, pointing up. No matter how you hold the book, the nail will point outward from its cover. If you know what direction the nail is pointing, you also know the orientation of the book. If you know the orientation of the book, you know where the nail is pointing. Thus, there is a duality between the nail and book.
So does the the nail determine the orientation of the book, or does the book determine the direction of the nail? That seems to be a nonsensical question. The duality between nail and book isn’t about one object determining the orientation of the other, but about the connection that exists between two of the nail-and-book’s fundamental properties, neither of which is caused [exclusively] by the other. You could say the same thing about quantum theory when someone asks if light is a particle or a wave.
first time i encountered a definition of #duality exactly like this one.