A brief history of the concept of sentience is given. It is pointed out that the idea of sentience, at least in the mammals and birds, was accepted by lay people by the time of the Renaissance and before it was acknowledged by philosophers. It was not until the Enlightenment of the 18th century that philosophers started to accept the notion that animals have feelings. Towards the end of the 19th century, scientists and philosophers had developed a fairly sophisticated concept of sentience. Little consideration was given to sentience by scientists through much of the 20th century due to the inhibiting influence of Behaviourism. In the last quarter of the 20th century, there was a surge of interest in animal sentience, and animal welfare scientists quickly realised that welfare problems can be better tackled with an understanding of how animals feel. Methods to investigate indirectly how animals feel are described and areas requiring further elucidation are listed.
Duncan, I. J. (2006). The changing concept of animal sentience. Applied Animal Behaviour Science, 100(1-2), 11-19. DOI: https://doi.org/10.1016/j.applanim.2006.04.011