  Fourth, the problem of determining whether animals are conscious  stretches the limits of knowledge and scientific methodology (beyond  the breaking point, according to some).  The so-called “cognitive revolution” that took place during the latter half of the 20th century has led to many innovative experiments by comparative psychologists and ethologists probing the cognitive capacities of animals.  The philosophical issues surrounding the interpretation of experiments to investigate perception, learning, categorization, memory, spatial cognition, numerosity, communication, language, social cognition, theory of mind, causal reasoning, and metacognition in animals are discussed in the entry on animal cognition. Despite this work on cognition, the topic of consciousness per se in animals has remained controversial, even taboo, among many scientists, while other scientists from a variety of disciplinary backgrounds (e.g. neuroscience, animal behavior, evolutionary biology) have developed novel ways of approaching the subject (see Boly et al. 2013 for a review).  The 2012  Cambridge Declaration on Animal Consciousness  indicates that many scientists agree that “the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness.” However, other scientists, including Marian Stamp Dawkins, who has been prominent in the science of animal welfare (Dawkins 1985, 1993), are not ready to endorse the claim, writing that, “The mystery of consciousness remains. The explanatory gap is as wide as ever and all the wanting in the world will not take us across it” (Dawkins 2012, pp. 171–172). 