I think of the carriage horses in New York City. I have no doubt that some or most of the people who work with these horses love them. But you can love someone to death. And can we really say that these horses "want" to work the hot streets for hours a day? Why would they enjoy this, when they are herd animals whose ancestors roamed freely with others of their kind, owing allegiance only to one another?
And if that is true, why stop there? What about zoos? How can you say that the animals in them have good lives when we lock them into enclosures thousands of times smaller than their natural habitat? For animals, mobility is freedom.
But we can go much further: does anyone, anywhere, believe that hens packed in tiny cages to lay eggs for us have a good or even tolerable life? Are these animals so robotically stupid that they have no idea what is happening to them? Of course not.
Cows, sheep, pigs, ducks, geese, rabbits. These are all animals who are exploited on farms for their milk, their eggs, their skin, their children, their flesh. Nobody, any longer, believes we do this for their sake. We do it for ours: for profit, for taste, for fun.
And, much as I hate to go there, what about dogs and cats? I love living with them and when they live with me and millions of people like me, they have a good life. But was it the life that nature intended them to live?
My dog Benjy would sometimes give me a look, when I gave him an order, that was part sadness, part defiance, part puzzlement: "Who are you to tell me what to do?" he seemed to say, and I felt shame.
We have evolved, possibly for the last 35,000 years, to be together and to enjoy being together, but is this the life Benjy evolved to live? Guide dogs, therapy dogs, hearing dogs - yes, they seem to love what they do, but is this enough grounds to justify what is, after all, a form of slavery?
Where will it end, you may understandably ask. I don't know. We are at a very unusual historical point in our relation to animals: Suddenly many are asking uncomfortable questions about a subject that we thought we already had mastered. We hadn't.
The time will come, I am sure, though I am not sure when, when we will look back at even the most benign forms of animal exploitation in horror tinged with sadness. "How," we will ask, "could we have been so blind to the suffering of others?" I am not sure what the answer will be, but I would love to be alive long enough to learn it.
Masson is author of "When Elephants Weep: The Emotional Lives of Animals" and other books.