Thesis Two: an animal is a being whose suffering should be of concern to humans.
Via vegan.com, The Human Society has a new video about the treatment of turkeys in industrial agriculture. A telling line from the Star Tribune article on the matter:
"Willmar [Poultry Co.] said much of what the video shows is acceptable industry practice..."
When we see video like this, we are not seeing unique aberrations of cruel indifference: we are seeing evidence of a system of cruel indifference.
And this is where things get fundamental: for animals to be treated the way animals are being treated in that video, people must either fail to see animals as conscious beings that can suffer, or they must fail to see the suffering of animals as something that should matter. It's quite obvious that a system in which this is "acceptable industry practice" fails to accept at least one of those two theses above. And for consumers/eaters who view such video, the consequence of accepting the above theses should evoke a desire to no longer support such a system.