A right is the recognition of an interest. To say that an interest is protected by a right is to say that the interest cannot legitimately be abrogated merely to benefit somebody else.
Inherent value and innate equality
The fundamental consequence of any recognition of rights is that we recognise a basic right not to be treated exclusively as the means to the end of another person. This is a basic right, and a prerequisite to any legal system of rights. If we don't have this right, then all other supposed rights become completely meaningless.
Another way of stating the right not to be treated as means to an end is to say that there is a right not to be considered solely as a resource for somebody else to use as they see fit. Without this right, no human being can be a legal or moral person within society. Without the right, the human being is not a person, but a thing.
If the value of a human being is only as a resource for others, then by definition the Principle of Equal Consideration (treat like things alike) is not applied. Thus, any logically defensible moral system must incorporate the Principle of Equal Consideration as part of its set of basic principles.
This concept of basic rights is well established in philosophy. For example, Immanuel Kant maintained that there is one innate, pre-legal right - the right of innate equality or "a human being's quality of being his own master". Kant said that this right "grounds our right to have other rights". Innate equality in turn demands that we apply the Principle of Equal Consideration.
An equivalent formulation of the idea that humans cannot be treated solely as the resources of others is to say that we recognise that all human beings, regardless of their personal characteristics, have inherent value beyond their value as a resource for other people. This is sometimes called the inherent or intrinsic value concept. Recognition of intrinsic value means we have a right not to be treated as a thing, but as a person. Things, as opposed to persons, only have extrinsic value - they are only valuable in so far as somebody else regards them as a valuable resource. Persons have intrinsic value.
Slavery as an example
Consider slavery. Slave owners were often advised to treat their slaves "humanely". Why? Not because of any rights of the slaves, but only due to the charity of the slave owners. Slaves are accorded no intrinsic value; their only value is as a resource for exploitation by their owners. An owner might treat his slaves humanely to protect his economic investment, but this is not a recognition of intrinsic value or rights of the slaves.
For example, in one slavery case in the USA, a court ruled that it had no jurisdiction to try a slave owner who beat his slaves with "rods, whips and sticks", even if the beatings were administered "wilfully and maliciously, violently, cruelly, immoderately and excessively". The slaves had no rights; they were the resources of the owner, to do with as he pleased. The only exception to this was if the beatings were administered in public rather than in private "not because it was a slave who was beaten, nor because the act was unprovoked or cruel; but because ipso facto it disturbed the harmony of society; was offensive to public decency, and directly tended to breach the peace. The same would be the law, if a horse had been so beaten." In other words, the court would consider the sensibilities of people who had rights not to be offended by witnessing a beating, but would not accord any rights to the subjects of the beatings themselves.
Basic rights vs. non-basic rights
The difference between a basic right and other rights is that non-basic rights may be sacrificed in order to secure basic rights, but not vice-versa. Sacrificing a basic right to secure other rights would be a self-defeating process, since no other rights can be truly enjoyed in the absence of the basic right. Those other rights would in fact be illusory.
To give another example, one commonly-cited example of a basic right is the right to physical security - the right not to be subject to murder, torture, rape or assault. It would make no sense to suggest that we sacrifice this right for a non-basic right, such as the right to vote. What use is being able to vote if you can't guarantee you won't be murdered?
The basic right not to be treated solely as a resource for others guarantees that humans cannot be bought and sold, used for biological experiments without their consent, killed and used to make clothing, hunted for sport, or killed and eaten.
Equal consideration and Animal rights
Now, consider animal rights. The Principle of Equal Consideration says that if we are going to take animal interests seriously and give any real content to prohibitions on inflicting unnecessary suffering, then we must extend the same protections to animal interests in not suffering as we extend to human interests, unless there is a good reason not to do so. Note that there is no middle ground. Either animal interests are morally significant, or animals are merely things which have no moral status. It might be economically "better" to not to treat animals cruelly, just as it might have been economically "better" to beat slaves only twice a week instead of five times a week, but this notion of "better" did not remove slaves from the category of "things".
Common excuses given for not recognising the equal inherent value in animals in not suffering, and therefore denying that the basic right of equal consideration applies to animals include: "Animals aren't as smart as humans", "Animals haven't claimed their rights in the way humans have". These are given as reasons to deny equal consideration to animals.
"Claiming our rights"
It has been argued that animals have no rights or intrinsic value because they have not "claimed their rights". This is one suggested reason for not applying the principle of equal consideration to animals, and therefore giving humans the right to treat animals as things, or as the sole means to ends of human beings.
This fails as a viable reason for denying equal consideration to animals because it sets up a double standard. Many human beings have never "claimed their rights". We recognise the rights of human children, disabled human beings and so on, not to be treated as things, but as beings who have intrinsic value. Yet a baby cannot fight to assert its rights. On the other hand, animals fight as best they can to assert their right not to be killed, for example. So, why regard animals as commodities yet regard babies as beings of intrinsic worth?
It has been asserted that membership of the human species is enough to give human babies basic rights. Human babies, even though they do not possess the special characteristic that is supposed to give rights (i.e. that they have "claimed their rights"), supposedly should still be treated as though they possess the required characteristic.
But this is simply begging the question. The issue here is to name a characteristic that ALL humans possess but which animals do not, which justifies us giving rights to all humans but to no animals. Pretending that all humans have a particular characteristic when some in fact do not, and when some animals may in fact possess the characteristic, doesn't overcome the inconsistencies of this argument.
The fact is, there is no special quality that all humans possess and that no animals possess, such as to justify the treatment of all animals as things, while all humans have intrinsic worth.
Some people argue that humans can talk, or do calculus, or write books, and that that makes the difference. Yet there are many humans who cannot talk, cannot do calculus, cannot write books. Should we therefore exclude those human beings who cannot talk or do calculus from the moral community? Clearly, we do not do that.
The Principle of Equal Consideration tells us that we ought to accord equal intrinsic value to all sentient creatures. Sentience in this context means having the ability to consciously experience pain and suffering.
Do animals consciously experience pain? Perhaps there are some of them that don't, but it is difficult to draw the line.
Does a dog feel pain the same way that a human feels pain? Again, it is difficult to know. But then again, we have the same problem with human beings. Do you feel pain the same way I feel pain? There's no way to tell for sure. And yet, we say all humans are sentient.
We know for our common meat animals that they have all the apparatus necessary to feel pain - a brain, a central nervous system etc. And they certainly act in ways that suggest they feel pain in the same way humans feel pain. So, if I am going to recognise that your pain, as another human being, is similar to mine, then I can't see any reason for imagining that a dog's pain is different in kind.
To be sentient is not the same as being alive. Sentience requires that you are the sort of being who has the capacity to be conscious of pain and pleasure. Consciousness, in turn, requires an "I" to have the subjective experience of pain.
Plants are alive but not sentient. They do not behave in ways that indicate they feel pain. They lack the neurological and physiological apparatus that is associated with pain in humans and animals. Moreover, they lack the need for pain as a survival mechanism.
The Principle of Equal Consideration would demand that we accord equal intrinsic value to plants and to humans in the matter of pain, unless there are compelling reasons not to do so. And there are. There is simply no evidence that a plant experiences pain at all. In fact, there's no evidence that plants have ANY conscious experience. There is no evidence that plants are an "I" which is conscious of itself as an ongoing entity.