With the advent of quantum mechanics, it appears that the world is irreducibly random. According to the standard interpretations of the theory, it is possible to set up an experiment with total control of all relevant parameters, which will still have a perfectly random outcome. Indeed, quantum mechanics does not predict the outcome of experiments, but only their probability distribution. Minority resistance to this idea takes the form of hidden variable theories in which the outcome of the experiment is determined by certain unobservable characteristics (hence the name "hidden variables"). The very existence of important classes of hidden variables would produce experimentally testable difference from what standard quantum mechanics predicts. See Bell test experiments.