"[B]y quantum field theory the dichotomy between matter and fields in the sense of a dualism is minimised as every field carries its particle-like quanta. Today’s unified field theories appear in the form of gauge theories; matter is represented by operator valued spin-half quantum fields (s) while the “forces” mediated by “exchange particles” are embodied in gauge fields, i.e., quantum fields of integer spin (s). The space-time geometry used is rigidly fixed, and usually taken to be or, within string and membrane theory, some higher-dimensional also loosely called “space-time”, although its signature might not be Lorentzian and its dimension might be 10, 11, 26, or some other number larger than four. A satisfactory inclusion of gravitation into the scheme of quantum field theory still remains to be achieved."