Bohr, one of the main architects of quantum theory, suggested, however, that no connection could be made between indeterminism of nature and freedom of will. Agent/substance-causal theories Agent/substance-causal accounts of incompatibilist free will rely upon substance dualism in their description of mind. The agent is assumed power to intervene in the physical world. Agent (substance)-causal accounts have been suggested by both George Berkeley and Thomas Reid. It is required that what the agent causes is not causally determined by prior events. It is also required that the agent's causing of that event is not causally determined by prior events. A number of problems have been identified with this view. Firstly, it is difficult to establish the reason for any given choice by the agent, which suggests they may be random or determined by luck (without an underlying basis for the free will decision). Secondly, it has been questioned whether physical events can be caused by an external substance or mind – a common problem associated with interactionalist dualism. Hard incompatibilism Hard incompatibilism is the idea that free will cannot exist, whether the world is deterministic or not. Derk Pereboom has defended hard incompatibilism, identifying a variety of positions where free will is irrelevant to indeterminism/determinism, among them the following: Determinism (D) is true, D does not imply we lack free will (F), but in fact we do lack F. D is true, D does not imply we lack F, but in fact we don't know if we have F. D is true, and we do have F. D is true, we have F, and F implies D. D is unproven, but we have F. D isn't true, we do have F, and would have F even if D were true. D isn't true, we don't have F, but F is compatible with D. Derk Pereboom, Living without Free Will, p. xvi. Pereboom calls positions 3 and 4 soft determinism, position 1 a form of hard determinism, position 6 a form of classical libertarianism, and any position that includes having F as compatibilism. John Locke denied that the phrase "free will" made any sense (compare with theological noncognitivism, a similar stance on the existence of God). He also took the view that the truth of determinism was irrelevant. He believed that the defining feature of voluntary behavior was that individuals have the ability to postpone a decision long enough to reflect