A proposal for network-assisted self-testing generalizes this approach to states of any number of qubits. How they appear seems to depend on how we choose to measure them, and before we measure they seem to have no definite properties at all — leading us to a fundamental conundrum about the nature of basic reality. Jump to: How is it different? Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large macroscopic scale. US: Springer. Metaphysics Research Lab, Stanford University. One can instead define reduced density matrices that describe the statistics that can be obtained by making measurements on either component system alone. Retrieved 18 May A fundamental feature of the theory is that it usually cannot predict with certainty what will happen, but only give probabilities. By quantum mechanics had been further unified and formalized by David Hilbert , Paul Dirac and John von Neumann [80] with greater emphasis on measurement , the statistical nature of our knowledge of reality, and philosophical speculation about the 'observer'.