  In contrast to Carruthers' higher-order thought account of sentience, other theorists such as Armstrong (1980), and Lycan (1996) have preferred a higher-order experience account, where consciousness is explained in terms of inner perception of mental states, a view that can be traced back to Aristotle, and also to John Locke. Because such models do not require the ability to conceptualize mental states, proponents of higher-order experience theories have been slightly more inclined than higher-order theorists to allow that such abilities may be found in other  animals[6].  Gennaro (2004) argues, however, that a higher order thought theory is compatible with consciousness in nonhuman animals, arguing that Carruthers and others have overstated the requirements for the necessary mental concepts and that reentrant pathways in animal brains provide a structure in which higher- and lower-order representations could actually be combined into a unified conscious state.