G. MacCallum's triadic scheme is helpful in understanding differences and similarities between the negative and hybrid views. Berlin becomes one of the founders of the hybrid view, by revising the definition of negative freedom into freedom as the absence of constraints 'to satisfy one's actual and potential desires and choices', in the new introduction he has written ten years after his defence of the negative view. The first version of hybrid theory introduces the possibility of an illusion of freedom that is absent in the negative view. The second version of hybrid theory has not emerged from an attempt to solve the paradox of the contented slave, which was the root of the first version. 'The capacity for reflective self-evaluation that is manifested in the formation of second-order desires' is the distinctive feature of humans. Watson makes another critique of Frankfurt's theory by referring to the problem of infinite regress.