Pomerleau write that if symbols and signals are the same thing, then "[s]ufficiency is a given, unless one is a dualist or some other sort of mystic, because physical symbol systems are Turing-universal." The widely accepted Church–Turing thesis holds that any Turing-universal system can simulate any conceivable process that can be digitized, given enough time and memory. Since any digital computer is Turing-universal, any digital computer can, in theory, simulate anything that can be digitized to a sufficient level of precision, including the behavior of intelligent organisms. The necessary condition of the physical symbol systems hypothesis can likewise be finessed, since we are willing to accept almost any signal as a form of "symbol" and all intelligent biological systems have signal pathways. Criticism The physical symbol system hypothesis applies only to systems with high level symbols, and not to signal processing systems (as found in most robots) or neural networks and other statistical machine learning methods. The criticisms below are only directed against symbolic AI, and not at these more modern forms of AI. Nils Nilsson has identified four main "themes" or grounds in which the physical symbol system hypothesis has been attacked. The "erroneous claim that the [physical symbol system hypothesis] lacks symbol grounding" which is presumed to be a requirement for general intelligent action. (Such as Brooks, below) The common belief that AI requires non-symbolic processing (that which can be supplied by a connectionist architecture for instance). (Such as Dreyfus, below) The common statement that the brain is simply not a computer and that "computation as it is currently understood, does not provide an appropriate model for intelligence". And last of all that it is also believed in by some that the brain is essentially mindless, most of what takes place are chemical reactions and that human intelligent behaviour is analogous to the intelligent behaviour displayed for example by ant colonies. Dreyfus and the primacy of unconscious skills Hubert Dreyfus attacked the necessary condition of the physical symbol system hypothesis, calling it "the psychological assumption" and defining it thus: The mind can be viewed as a device operating on bits of information according to formal rules. Dreyfus refuted this by showing that human intelligence and expertise depended primarily on unconscious instincts rather than conscious symbolic manipulation. Experts solve problems quickly by using their intuitions, rather than step-by-step trial and error searches. Dreyfus argued that these unconscious skills would