Can animal rights be justified from a libertarian position? In Animal Rights for Libertarians,  Jeremy R. Garrett notes that of all the major ethical theories, libertarianism stands as an exception for, not merely not having elucidated a theory of moral rights for animals within its framework, but often rejecting animal rights as being compatible with their position. To tackle this problem, Garrett offers an argument that Reagan’s deontological rights based approach fits “exceptionally well”  within Nozick’s theory of libertarianism.
To begin, Garrett identifies three parts fundamental to libertarian framework. First, libertarianism offers “a general account of the nature of (enforceable) moral constraints” ; second, it offers “a view about moral status and the properties of beings or entities that make moral constraints applicable to them” ; and third, it offers “a specific account of the practical implications of these constraints.” 
On the nature of moral constraints, Garrett identifies the central moral concept of libertarianism as moral rights, rights which function as “’side-constraints’ that limit the initiation of non-consensual force against others.”  In essence, these moral rights require the following. First, that violating moral rights is impermissible, whether done for the sake of consequences (i.e. they are independently valid), or even to reduce other rights violations. Second, that these rights are negative, meaning that they constrain actions that can be taken toward the rights holder, as opposed to positive rights, which would confer some duty to a rights holder on behalf of other individuals. And third, that the duties moral rights impose upon moral agents (i.e. to refrain from initiating force) are enforceable, meaning that “force may be used ‘to ensure compliance.’” 
Garett next discusses moral status within a libertarian framework, explaining that having status entails that a being has moral claims on “those who can recognize such claims.”  In order to determine which beings have moral claims, we must analyze the characteristics held by individuals that will entail such claims. Garett notes that within libertarian frameworks, there are generally two theoretical constraints involved with determining which beings validly have rights. First, that all and only non-human animals have rights. Second, that the basis for assigning “rights-worthy moral status” must “respect the separateness and inviolability of individuals.” Hence, the characteristics chosen to confer moral status tend to be those which “imply that their possessor has a distinctive and inherently valuable existence independently of the interest of any other being or entity.” 
To finalize this section, Garrett discusses some of the implications of the libertarian view of rights, noting that while some follow uncontroversially from the above, other positions held by libertarians are controversial. On the former, Garrett points to the libertarian tradition for “strong protections of life, liberty, and bodily integrity,”  but notes on the latter that the libertarian position with respect to the acquisition of property remain controversial. However, Garret notes that for the sake of the argument he will present, only the former aspects of libertarianism will be relevant.
Having detailed then the nature of moral rights, their constraints, and the nature of status, Garrett suggests that one need only add one qualification to the above in order to secure rights for animals. Namely, that the criteria which confers rights incorporate Reagan’s “subject of a life” criteria, which Garrett notes is a “sufficient condition for being viewed as a distinct ‘individual with inherent value equal to that of all other beings … ‘”  If done so, then at the very least animals will be conferred the aforementioned libertarian rights of life, liberty, and bodily integrity.
This conclusion serves as the first premise of the syllogism introduced next, a brief version of which is offered here, which Garrett refers to as “Animal rights for Libertarians”:
1. If a being satisfies Reagan’s “subject of a life”
criteria, that being is entitled to rights to life liberty, and bodily
p2. Some non-human animals are subjects of a life
c. Therefore some animals are entitled to these rights
Following the introduction of this syllogism, Reagan entertains two potential libertarian objection to ARL. The first would reject the first premise and argue that Reagan’s criteria are not sufficient for a being to be granted rights, and the second would reject the second premise and argue that no non-human animals meet Reagan’s criteria.
To tackle the first objection, Garrett considers first Nozick’s argument on characteristics in Anarchy, State, and Utopia, wherein Nozick sets out criteria that seem plausible as candidates for being “individuating characteristics connected with moral constraints.” However, Garrett notes that Nozick goes on to argue that he recognizes the is-ought gap with regard to these characteristics, which is to say that deriving from these descriptive facts a set of oughts is problematic, whether considered individually or jointly. And considering that the criteria Nozick offers goes beyond the criteria offered by Reagan, if Nozick holds a hyper skeptical position on the former, he is unlikely to be impressed by the latter. Garrett’s move here is not to accept Nozick’s hyper skepticism, but rather to temper it and proceed in his argument (for surely such hyper skepticism arrests most if not all ethical theorizing if such theorizing is to be grounded in fact), noting that “his libertarianism is fundamentally rooted in a view of moral rights … which reflect the separateness of individuals.” 
Proceeding then, Garrett argues that libertarians should accept Reagan’s subject of a life criteria as sufficient to secure moral status because to be a subject of a life clearly distinguishes individuals from one another, and that “what happens to a subjects-of-a-life matters to them irrespective of its consequences for anyone else—they are subjectively concerned for their own good and take actions to pursue it.” And if this individuating subjecthood is not sufficient criteria to be granted moral rights, Garrett rhetorically ponders what criteria moral status might require as, after all, Nozick has rejected the more extensive criteria he himself offered. But for Garrett, Nozick’s hyper skepticism misses a bigger point: that this set of criteria leave libertarianism in a dilemma.
On the one hand, some of the criteria offered, such as possessing free will and having a soul, are unverifiable. As such, it is impossible to determine which beings should be conferred rights. For libertarians who wish to grant these rights to humans and not-animals, this problem is clearly vicious. On the other hand are a set of properties such as sentience, rationality, and moral agency, which are “not possessed by all and only humans,”  which entails, as Garrett notes, that either some humans must lack status or some non-humans will hold status. Thus, if libertarians are to deny animals rights, they must discover a property that is verifiable, sufficient to confer rights, and which is shared by all humans, but no non-humans. As it happens, this is exactly what Nozick attempted to do in a publication ten years subsequent to Anarchy.
Nozick’s later argument, in essence, eschews the particular criteria offered previously, and instead grounds moral status in what Garrett calls a “milieu,” which is to say that just grounds moral status in the overall context in which a being resides, with humans being part of a cultural, familial, and historical web to which animals are not subject. Being part of this milieu is offered as a necessary condition to hold rights, and as such, if valid and sound, would preclude animals from holding them. Garrett however takes exception to Nozick’s claim on three grounds. First, because the milieu criterion is offered as a necessary criteria, Garrett notes that it has implications on the moral status of embryos and fetuses, which is to say that they might not meet the criteria, and hence such a grounding would be unappealing to many libertarians. Second, that such a criteria, due to its ambiguous nature, may easily be appropriated by subgroups within a population who wish to exclude all those outside of their particular milieu from moral status (e.g. white supremacists). Third, Garrett criticizes the ambiguous and perhaps unjustified nature of the “we” who gets to decide the moral status of animals vs. humans in Nozick’s definition as follows:
We see humans, even defective ones, as part of the multifarious texture of human history and civilizations, human achievements, and human family relations. Animals, even year-old mammals, we see against a different background and texture. The differences are enormous and endless.
Garrett first attempts to clarify the identity of “we” by positing that perhaps it is empirical in nature, and represents the “collective judgment of all or most human beings.”  He rejects that “all” is plausible by noting that many human beings have historically not seen, and contemporarily don’t see, all other humans as part of their shared culture, texture, or background. Further, many people do incorporate animals into their particular milieu (e.g. animal companions). Next, Garrett rejects the plausibility of a “most” human beings position, noting first that this empirical claim as such has no data to support it, and that intuitions in such a case would be unreliable. But regardless of whether Nozick means “all,” or “most,” would such an empirical truth plausibly ground rights for humans and render animal rights non-existent?
Garrett thinks it unlikely, principally because this grounding runs into the same is-ought problem as Nozick’s ten years previously offered criteria. But in addition, the logic behind Nozick’s correction runs into further problems. For one, historically, many indefensible views have been held by a majority of the human population, if not the human population in toto (e.g. slavery). As such, Garrett cautions us to approach the views of our own epoch with such confidence if they result in the harm of some group. Second, Garrett notes that whether a view is valid or not is predicated not on the acceptance of that belief by some portion—majority or otherwise—of a population, but whether the view itself is independently supported by good reasons. Garrett concludes this section by noting that without a normative defense for the acceptance of Nozick’s milieu view, we should reject it.
In the next section, Garrett analyzes a possible rejection of p2 (some non-human animals are subjects of a life), which, as an empirical premise, would require an empirically based rejection—one that Nozick offers. Nozick makes two claims regarding animals as subjects of a life: first, that Reagan’s view on the complexity of animal life is inaccurate, and that it may better be explained simply by Skinnerian operant conditioning; and second, that even if animals do possess characteristics analogous to humans, they “are far more rudimentary.” In response to the former, Garrett notes that, for one, in the time since Nozick wrote his article, scientific researchers have discovered significant evidence with regard to the complexity of the animal experience of, e.g., pleasure and pain, enjoyment and suffering, etc.; and second, that Nozick does not offer evidence that Skinnerian operant conditioning does in fact hold explanatory power. On the latter, Garrett merely rhetorically asks whether, even if it is the case that animals are far more rudimentary, is the difference between us big enough such that “more sophistical beings [are justified] in killing or harming more rudimentary beings.”
With this said, Garrett notes that these objections are merely critiques contingent upon the particular methods Nozick used to criticize Reagan’s subject of a life criteria, and that at its core, Nozick’s argument is more simply one that rejects the empirical basis of Reagan’s claims. In response to this more general argument, Garrett notes that the ideal means of argumentation would be sampling a comprehensive review of the literature. But for the sake of space, Garrett focuses on a less ideal, but perhaps still compelling, means of argumentation. He appeals, first, to common sense, arguing that extraordinary evidence would not normally be demanded in a good faith discussion on an analogous situation, like that of an infant child. That is, a common sense view is that animals are individuals with “beliefs, desires, memories, emotions, [with] a distinct perspective of their environment, [and] a welfare of their own.”  But, if someone were to ask for a justification in good faith, what could one offer? Garrett’s response to this line of questioning draws on the shared evolutionary history of human and non-human animals, an evolutionary history that results in these respective kinds having similar biological systems, and hence similar experiences. But perhaps Nozick might reject this argument as well, and fall back instead on the previous assertion that even if animals and humans do share some general experience in common, that of the animal is far more rudimentary. If this is the case, then Garrett notes that we appear to be at a stalemate.
In dealing with this stalemate, Garrett returns to an argument mentioned in the dilemma above: if the differences between humans and animals are significant such that humans are justified in denying moral status to animals, is it not consistent with this position to deny moral status to humans who fall far below normal human standards? If not, then the libertarian position toward animals is clearly not consistent. Another route Garrett takes is to propose a common means of dealing with normative questions when the underlying empirical evidence is in dispute. Simply put, because libertarians hold as its moral core the protection of life, liberty, and bodily integrity, the burden of justifying action toward some potential member of a protected group should fall squarely on the one who wants to harm an individual with a central nervous system. Said another way, we should be cautious in our action toward any individual who might hold rights worthy status, rather than beginning with the premises that we have a right to harm until proven otherwise.
Garrett concludes his argument by noting that the trend in animal research thus far has been to discover that our understanding of them is simple with respect to how complex they really are, and that as research continues, we should not be surprised if our understanding of their complexity grows as well.
 Garrett, Jeremy R. “Animal Rights for Libertarians.” The Moral Rights of Animals. By Gary Comstock and Mylan Engel. Lanham: Lexington, 2016.