The moral circle - the set of beings whose interests receive moral consideration - has expanded historically and continues to expand. Understanding this expansion helps advocates present animal welfare as part of a continuous moral progress.
The moral circle expansion argument provides one of the most powerful historical cases for animal welfare advocacy. Each previous expansion - to people of different races, to women, to distant peoples - was initially resisted with arguments that those seeking inclusion were fundamentally different and their interests appropriately discounted. In retrospect, these resistances appear morally unjustifiable. The argument is not that animals are identical to humans but that the historical pattern suggests we should be suspicious of confident denials of moral consideration to large groups of sentient beings. The trajectory of expanding moral consideration, driven by growing recognition of shared capacity for suffering, argues for including animals within genuine moral concern.