How Many Dimensions are Required in Physics ?

The different concepts of "space" are contrasted which have been developed in mathematics and in physics. Early proposals of extra dimensions are briefly reviewed. The main thesis claims that in physics not the dimension number of the underlying space is essential ; rather, there are strong reasons to use vector spaces over the field of complex numbers, not strictly excluding other structures. Reasons for this come both from mathematical rigour and from human cognition. Applications to physics concern nonlocal processes, conditions for quantum entanglement, and a proposal of hidden organizing structures. An experimental test for nonlocal hidden structures is proposed.