This paper examines reasons why animal existences can be regarded as subjects of moral concern. This debate is examined in conjunction with contemporary discussion on this issue, with
Aristotle's thinking on animals in the background. The change in thought brought about by thinking on sentience is taken account
of. The issue whether animals are moral agents like humans, as argued in Aristotle and contemporary thinkers, is addressed. In particular the recent views of Bekoff and Cohen are examined. With reference to Irvin and Bekoff, the moral relevance of cognitive capacities in animals is considered. The article concludes that higher capacities, especially self consciousness,
are indeed morally relevant to the issue.