Early views on the capacity of animals to experience pain and other sensations were often predicated more on philosophical positions than on scientific observation. Followers of the mind-body dualism of Descartes denied the existence of mental states in non-human organisms. The Romantic tradition of the 19th century attributed elaborate anthropomorphic thoughts, feelings and intentions to animals. Behaviorists of the early 20th century side stepped the issue: because psychological states were private, they could not be characterized objectively, even in humans. Recent evidence regarding subjective experience in animals comes from neurophysiological and ethological (behavioral) studies (e.g., Lorenz, 1971). Physiological evidence indicates that animals which possess a central nervous system, or which show evidence of receptors for endogenous opioids, have the potential to experience pain. Behavioral evidence of pain in many higher vertebrates is similar to its manifestations in humans, including screaming, squealing and struggling. Behavioral evidence of pain in species more remotely related to humans (e.g., fish), and of less obvious forms of distress like fear, frustration, exhaustion and anxiety in all nonhuman animals is more difficult to identify.