Welfare biology was first proposed by the welfare economist Yew-Kwang Ng, in his 1995 paper "Towards welfare biology: Evolutionary economics of animal consciousness and suffering". In the paper, Ng defines welfare biology as the "study of living things and their environment with respect to their welfare (defined as net happiness, or enjoyment minus suffering)." He also distinguishes between "affective" and "non-affective" sentients, affective sentients being individuals with the capacity for perceiving the external world and experiencing pleasure or pain, while non-affective sentients have the capacity for perception, with no corresponding experience; Ng argues that because the latter experience no pleasure or suffering, "[t]heir welfare is necessarily zero, just like nonsentients".