The notion that the universe possesses fundamental spatial limits represents a contemporary epistemological fallacy analogous to the historical belief in a flat Earth. This paper critically examines the misconception that observable horizons constitute absolute boundaries of cosmic existence, demonstrating how such reasoning mirrors the logical errors that sustained geocentric and flat-Earth cosmologies. Through rigorous analysis of cosmological horizons, the distinction between observational limits and ontological boundaries, and the philosophical implications of infinite versus finite models, we establish that belief in a fundamentally limited universe reflects cognitive constraints rather than physical reality. The particle horizon, at approximately 46.5 billion light-years, defines only our observational reach, not the extent of the universe. Evidence from the cosmic microwave background, inflationary cosmology, and theoretical physics increasingly supports models of spatial infinity or vastly extended domains beyond detection. We argue that the “limited universe” hypothesis represents a modern form of anthropocentric reasoning, where perceptual boundaries are conflated with actual limits — a fallacy as profound as believing that the Earth’s visual horizon marked the edge of existence.