Two-dimensional does not necessarily mean two spatial dimensions. It is dimensionality, or orthogonality, in the most abstract sense. For all practical purposes, complex numbers represent a two-dimensional system.
It is not a coincidence that they arose in 16th century Italy in the context of "completing the square" and related 2D methods/intuitions for solving equations.
It is not a coincidence that they arose in 16th century Italy in the context of "completing the square" and related 2D methods/intuitions for solving equations.