In this essay, we will look at the moral relevance of sentience as the characteristic justifying a basic right to physical security (as defined in the next section), but first we should cover some preliminary notions which will set the stage for understanding how and why sentience matters when we are thinking about animal rights.A Brief Introduction to Rights
A right is a way to protect an interest. Rights claims can be expressed as follows: A has a right to X against B by virtue of Y. A is the right holder; X is the interest protected by the right; B is the duty bearer; and Y is the relevant characteristic giving rise to X (i.e. the interest protected). We can also say that Y is the reason A has a right to X.
As there are many interests which vary widely in degree of relative importance from crucial to trivial, so there are many “rights” which vary in degree of relative importance in relation to the interest they are protecting. A right, the way the term is used in our society, can be as important as our basic right not to be tortured and killed or as trivial as our non-basic “right” of first refusal to buy an inexpensive and relatively unimportant item that a seller has set aside for us. It is therefore important to distinguish between basic rights and non-basic rights. In Introduction to Animal Rights , Gary Francione quotes the political theorist Henry Shue in Basic Rights  in describing a right as basic if “any attempt to enjoy any other right by sacrificing the basic right would be quite literally self-defeating, cutting the ground from beneath itself.” Shue also states that “non-basic rights may be sacrificed, if necessary, in order to secure the basic right. But the protection of a basic right may not be sacrificed in order to secure the enjoyment of a non-basic right” because a basic right “cannot be sacrificed successfully. If the right sacrificed is indeed basic, then no right for which it might be sacrificed can actually be enjoyed in the absence of the basic right. The sacrifice would have proven self-defeating.” 
The most important basic right Shue identifies is the “basic right to physical security – a right that is basic not to be subjected to murder, torture, mayhem, rape, or assault.”  As Francione says on page 95, “If a person does not enjoy the basic right to security, and may be murdered at will by any other person, then it is senseless to consider what other “rights” she might have.”  Of course, another basic right which is essential to a basic right to physical security is the right not to be the property of another; however, the basic right not to be property is beyond the scope of this essay. I will discuss the property status of nonhuman beings and their right not to be property in a future essay.
Sentience, narrowly defined, is the ability to experience, or to be conscious of, sensations. Sensations include pain, pleasure, sight, hearing, taste, and smell. More broadly defined, sentience includes the experience of a self. The experience of self which defines sentience is emotion- or sense-based and non-cognitive. In this essay, I mean sentience in the broad sense which includes a non-cognitive sense of self.
Sentience often varies in degree. If we are on pain killers or a substance which makes us less alert to our surroundings, our sentience is not as acute as when in a drug-free state. Also, some species have greater degrees of sentience with respect to specific senses than other species do. For example, dogs, pigs, and bears have a heightened sense of smell compared to most other species, and many birds have a comparative heightened sense of sight.
People often ask if insects are sentient. I don’t know to what extent they are. The question of where to draw the line on sentience, particularly its degree, is a difficult and lengthy topic to cover, and I will not address it in this essay. What we do know for certain is that birds and mammals are sentient in a way and to a degree highly similar to humans; so much so that any differences in sentience are morally immaterial. We have good reason to believe that other vertebrates, such as fishes, reptiles, and amphibians are also sentient to a high degree; although as we get further from biological similarities to humans, such as in the case of insects, it gets more difficult for us know what a being’s sentience or experience is like, in kind or degree.
An Interest in Physical Security 
Very closely related to sentience is a being’s interest in physical security. In fact, the only way we can determine whether an organism has an interest in physical security is to ask whether and to what degree the organism is normally sentient (or potentially sentient). A tree is alive, but because a tree has no sense organs or any other apparatus which might lead us to believe that it is sentient, nor does a tree behave outwardly in any way indicating sentience, we think it quite obvious that the tree cannot have any conscious interest in its physical security (since it cannot experience its physical security). On the other hand, chickens, pigs, cows, deer, sheep, goats, and many other nonhuman beings do have sense organs and well-developed central nervous systems, which, along with the outward behavior of fight or flight when facing perceived danger, cause us to know, to the same degree that we know with respect to humans, that they all have a strong conscious interest in their physical security.
So there is an obvious and strong connection between sentience and a conscious interest in physical security such that, to whatever degree there is sentience, there is a conscious interest in physical security; and to whatever degree there is a conscious interest in physical security, there is sentience.
Back to Rights
We said that a rights claim can be expressed as follows: A has a right to X against B by virtue of Y. Let’s fill in the terms: Sentient nonhuman beings (A) have a right to physical security (X) against human beings (B) by virtue of their sentience (which is virtually interchangeable with a conscious interest in physical security) (Y). We should note, and will explore more in the next section, that sentience is the same reason and the only reason why humans have a right to physical security.
Irrelevant Substitutes for Y
Self-interested animal exploitation advocates often attempt to suggest characteristics other than sentience for Y. As I stated in “Understanding the Anti-Animal Rights Viewpoint”, animal exploitation advocates work backward from the conclusion that they want to exploit nonhuman beings and look for “premises” to “support” their preconceived, self-interested conclusion. So animal exploitation advocates think up characteristics that only humans possess to substitute for Y and insist that their selected (irrelevant) Y is the “justification” for humans possessing the basic right to physical security, but since nonhumans don’t have that Y, nonhumans don’t possess that basic right.
The objections to animal exploitation advocates are obvious in two ways, both of which we’ll explore in more detail below. First, the suggestions that animal exploitation advocates come up with for Y are irrelevant to a conscious interest in physical security. Second, animal exploitation advocates’ suggestions for Y, with only one exception, are not characteristics which all and only humans have, and therefore exclude millions of humans from the moral community. The one exception is human DNA, or species, per se; which is as irrelevant to an interest in a basic right to physical security as race or sex, for example, are to an intelligent and curious human’s interest in education.
Let’s unpack the first objection that the suggestions for Y are irrelevant. A very common suggestion is cognitive ability (sometimes referred to as “rationality” or “intellect”). How or why cognitive ability connects to a conscious interest in physical security is far enough beyond me to seem silly, but I’ll play along by acknowledging a couple of facts which may make it clear to the self-interested exploiter that cognitive ability is a dead-end possibility for Y in this case.
The first fact to acknowledge about cognitive ability is that it varies widely from human to human. Some humans are mathematical, linguistic, and logical geniuses, while other humans are functionally lower than dogs, pigs, and turkeys in cognitive ability, while the rest of us fall somewhere in between the two. Does this mean that the mathematical geniuses have a more significant conscious interest in physical security than the less cognitively endowed humans? No, it doesn’t. A high level of cognitive ability may cause us to dread the future or realize that the future will be fine, but a lack of such cognitive ability may cause similar dread or comfort, depending on the situation. For example, a human may dread the possibility, during wartime, of being captured and tortured by an enemy army or political faction which never actually happens; and a rescued, formerly tortured farmed animal or canine may dread her sanctuary or shelter rescuers for several weeks or months not knowing that the rescuers will show nothing but sympathy and kindness toward her. It is the emotional and sentient activity that we have in common with nonhumans, not the abstract reasoning activity, from which our interest in physical security comes.
The second fact to acknowledge is that today’s computers have mind-boggling mathematical and logical ability, but absolutely no conscious interest in their physical security. If exploiters want to assert intelligence for Y, then insentient and unconscious, but highly intelligent computers get a right to physical security (even though they cannot experience physical security); and the much less intelligent, but sentient humans go without such a right.
Other suggestions for Y are the ability to assert or defend rights and the related ability to reciprocate morally; but again, that leaves millions of humans without rights. The ability to defend rights also possibly grants rights to anyone who asserts or defends their “right” to commit genocide, rule as the world’s tyrant, or any number of other such absurdities, to which the sane among us agree that no such rights exist. The ability to reciprocate the observance of a basic right to physical security only becomes relevant if our own physical security is immediately and actually threatened; and if such security is threatened in this way, the capacity of our attacker for moral agency becomes irrelevant anyway. We are at least excused, if not justified, in defending ourselves from anyone, moral agent or moral patient, who immediately threatens our physical security using the minimum force necessary, and such a defense at times can include killing. 
The second objection against animal exploitation advocates is that there are no characteristics which all and only humans have which differentiate us from other species in any morally relevant way to a conscious interest in physical security. In fact, there is an overlapping continuum of all characteristics, except DNA (which is irrelevant for reasons already stated), such that for any given characteristic, there are nonhuman beings who hold that given characteristic (e.g. intelligence, emotion, sensitivity to pleasure and pain, etc) more than some humans.
It should be clear by now to any coherent and reasonably intelligent reader that sentience is the morally relevant characteristic for the right to physical security. When we are in a position of power over others, the choice of whether or not to act morally, as in all cases of morality, is our choice to make. But the question of what is moral and what is not moral is not our choice in such clear cases; and it is a matter of moral fact that sentient nonhumans have a basic right against humans to their physical security in virtue of their sentience, regardless of how we happen to feel about that fact. Survival situations may excuse us in overriding the rights of others, but in our exploitation of nonhuman beings in our modern era, nothing even remotely similar to survival is at stake.
This inevitable conclusion, combined with our modern knowledge of vegan nutrition and alternatives, entails a moral obligation of ours to go and stay vegan. Fortunately, being vegan is much easier and far more delightful than most people imagine it to be. We just need to learn the ropes and develop the habits. We’re on autopilot after that.
 Gary Francione, Introduction to Animal Rights: Your Child or the Dog (Temple University Press, 2000), pages 94 and 95
 Henry Shue, Basic Rights, 2d Ed. (Princeton University Press, 1996)
 Although sentient beings have an interest in absolute physical security, some violation of physical security is unavoidable, inevitable, and therefore “necessary” for all of us (e.g. car accidents and disease). It does not make sense to talk about a right or interest in physical security if the breach of physical security is really unavoidable. I have therefore limited the discussion to an interest in physical security where it is protectable, but it should be clear that I mean “protectable” in the strong sense as protectable even at a very high cost, actual or perceived. Given this definition of protectable, it should be clear that somewhere around 99.999999% of our society’s intentional uses of nonhuman beings violates their protectable interest in physical security, and are therefore violations of their right.
 Upon re-reading this paragraph the day after it was posted, I decided it wasn’t written as clearly as it could have been. It is clarified below by changing/adding language as follows (changes are in bold):
“The ability to reciprocate the observance of a basic right to physical security only becomes an issue if our own physical security is immediately and actually threatened, since if our security is not immediately threatened, our right to security is not being violated; and if such security is threatened in this way, the capacity of our attacker for moral agency becomes irrelevant to our response anyway. This is because we are at least excused, if not justified, in defending ourselves from anyone, moral agent or moral patient, who immediately threatens our physical security using the minimum force necessary, and such a defense at times can include killing.”