What is the difference between humans and animals? Biologically or psychologically, the differences are immediately apparent: humans have unmatched language, rationality, adaptability, cooperation, and accomplishments. However, less obvious to many is the existence of a moral difference between humans and other animals.
Many argue that humans are not morally distinct from animals because their physical differences, such as a higher brain mass to body mass ratio than those of other animals, do not warrant moral distinction. Others argue that these same physical differences result in significant differences in morally relevant qualities like consciousness or knowledge.
On the spiritual side, some argue that spiritual qualities shared among all animals render even the most physically significant differences morally insignificant. Others believe that only humans are endowed with certain spiritual qualities, like souls, that make them morally distinct from animals.
As you can see, a lot of disagreement on this question hinges on disagreements over what is morally significant. What is your criteria for determining moral significance? What is the value that your criteria leads you to place in animals? How does this value compare to the value that you place in humans?