 Several psychologists and ethologists have argued for the existence of animal consciousness by describing a range of behaviors that appear to show animals holding beliefs about things they cannot directly perceive—Donald Griffin's 2001 book "Animal Minds" reviews a substantial portion of the evidence.Animal consciousness has been actively researched for over one hundred years.In 1927 the American functional psychologist Harvey Carr argued that any valid measure or understanding of awareness in animals depends on "an accurate and complete knowledge of its essential conditions in man".