1. All animals are equal

2. Thesis
Singer’s thesis: We ought to extend to all animals the same moral consideration that we extend to human beings.

3. The Argument
The only criterion of moral importance that we use to justify what we do to animals is simple membership in the species Homo sapiens.
However, using simple membership in the species Homo sapiens as a criterion of moral importance is completely arbitrary.
Of the remaining criteria we might consider, only sentience is a plausible criterion of moral importance.
Using sentience as a criterion of moral importance entails that we extend to other sentient creatures the same basic moral consideration (i.e. “basic principle of equality”) that we extend to human beings.
Therefore, we ought to extend to animals the same equality of consideration that we extend to human beings.

4. Details
1. The only criterion of moral importance that we use to justify what we do to animals is simple membership in the species Homo sapiens.
Like it or not, we must face the fact that humans come in different shapes and sizes:
People come with differing moral capacities.
Differing intellectual abilities.
Differing amounts of benevolent feeling and sensitivity to the needs of others.
Differing abilities to communicate effectively
Differing capacities to experience pleasure and pain.
So? If the demand for equality were based on the actual equality of all human beings, we would have to stop demanding equality!
If you still want to say that every human is morally important, and that humans are the only creatures that are morally important, then your criterion for moral importance is based simply on species membership.

5. 2. However, using simple membership in the species Homo sapiens as a criterion of moral importance is completely arbitrary.
If there is no relevant difference between your group and some other group, then there is no rational ground for thinking that your group deserves greater consideration than another.
Obviously, it is natural for people to use species membership as a criterion of moral importance.
After all, “Human beings are special” is often an empty phrase uttered by, well, human beings!
Singer points out the obvious parallel with racism. (My race is superior to yours)

6. 3. Of the remaining criteria we might consider, only sentience―the capacity of a being to experience things like pleasure and pain―is a plausible criterion of moral importance.
Singer argues for this in two ways:
First: The other criteria are bad because exclude people who we think ought not to be excluded.
For example: many people claim that the well-being of animals is unimportant because animals are not as intelligent as humans.
However, people with very late-stage dementia, the senile, even infants are less intelligent than some animals.
Therefore, intelligence is not a plausible criterion of moral importance.

7. Second: Singer argues that it is only by virtue of being sentient that anything can be said to have interests in the first place:
“The capacity for suffering and enjoying things is a prerequisite for having interests at all, a condition that must be satisfied before we can speak of interests in any meaningful way”(175).
Common criticism of Singer: This criterion ends up excluding humans who are no longer sentient (like those in an irreversible coma, Alzheimer’s, etc.). Singer bites the bullet.
Our recognition that it is bad for us to suffer automatically leads us to conclude that suffering is bad in the first lace.
If suffering is bad, then it must be bad for any creature to suffer.
If we wish to say that our suffering is more important than the suffering of others, the burden of proof is on us.
Looking again at the arguments above, it is clear that Singer thinks this burden of proof cannot be met.

8. 4. Using sentience as a criterion of moral importance entails that we extend to other sentient creatures the same basic moral consideration (i.e. “basic principle of equality”) that we extend to human beings.
Let’s be clear: Singer is not saying that we are required to give animals equal rights.
Elephants should not be given the right to drive a car. Squirrels should not be allowed to own a highway.
Rather, Singer argues that the capacity to feel pain/pleasure confers upon an individual equal moral consideration.
Thus, when we fight for equality of all human beings, we cannot possibly mean that all human beings are in fact equal. Right?
Then by virtue of what do we argue that all humans are equal?
Obviously, by virtue of the capacity to feel pain/pleasure—in short, sentience.
Consequently, accepting this criterion (and as we have seen it is the only objective and consistent criterion) if follows that we must extend equal moral consideration to all sentient beings, cows, pigs, dogs, chickens, etc.

9. Not convinced?
Think about it: Why is slavery wrong? Is it because they are Africans? Is it because they are humans? Is it because they are intelligent? Is it because they can talk? Or is it because they can suffer and slavery causes suffering? It seems clear then that what’s morally important (and gives us moral importance) is the capacity to feel pain/pleasure— sentience.

10. Personhood
Singer defines a person as a creature aware of its own persistence over time. Personhood is not equivalent to humanity: Singer argues chimpanzees are persons, but that human infants are not. Because they are aware of their own existence over time, persons are vulnerable to special forms of suffering. So, taking into equal consideration the comparable interests of all sentient beings does not necessarily force us to treat all sentient beings equally. If, for instance, one can save either a dog or a human adult from a burning building, Singer would say you must save the human adult, because the balance of interests satisfied will be greater if you save the adult, than if you save the dog.

11. However, Singer argues, situations where one is faced with that kind of choice are unusual. Most of our practices toward animals, however, are very easy cases to Singer: in the vast majority, we sacrifice important animal interests (such as life and freedom) for the sake of absolutely trivial human interests (such as satisfying a taste for meat). The suffering inflicted on factory farm animals vastly outweighs our enjoyment of cheap and convenient meat. All such practices, Singer concludes, quite clearly lack any moral justification, and must be eliminated as quickly as possible.

12. The scope of livestock slaughter
The following are the United States livestock and poultry slaughter statistics for the year of 2014 from the USDA National Agricultural Statistics Service:
30.2 million cattle 565,800 calves
106.9 million hogs million sheep and lambs
8.66 billion chickens million turkeys
26 million ducks

13. The truth
If one agrees with Singer’s argument, these statistics offer a glimpse of a moral problem so immense that it must dwarf virtually any other moral problem imaginable. Think about it: The slaughter of billions of animals is justified on the basis of taste for meat!