The current trend in liberalism is to extend rights to animals as in "animal rights. Most liberal jurisdictions have humane laws. Kosher and halal injunctions also relate to prohibition of animal cruelty. Buddhists include all beings. So do so-called primitive peoples.
The idea that reason can be separated from biology and ontology is a Western notion that arose in ancient Greece with the assumption that the rational principle distinguishes humans from non-humans. This was expressed philosophically and theologically through the concept of the human soul. In Christian doctrine, only humans had souls. This gave rise to the René Descartes assuming a separation of mind and body, as well as that animals were machines without souls.
This led to the Western liberal concept of the person as distinct from the individual. While human individuals are unique and unequal "accidentally," human "personhood" makes all humans equal as persons, meaning that there is no innate privilege through birth, so all are equal before the law and all enjoy equal rights as human beings, although under bourgeois liberalism, "all" was initially defined to exclude women and those to of European ethnicity.
This concept and its associated assumptions was questioned by science, which could find no material basis for it. Humans came to be viewed as machines without souls, like other animals. This became the basis for behaviorism in psychology, which was imported into economics as a fundamental assumption called "rationality"as "maximizing utility." Think Pavlov's dogs, Skinner's pigeons and lab rats. This was not lost on marketing & advertising, and PR.
Psychology and the social sciences are moving beyond that view based on discoveries in biology and neurology. Evolutionary theory is also refuting the Cartesian assumptions as well. See, for example, Antonio Damasio, Descartes' Error.
The Enlightened Economist
Not the smartest animals
Diane Coyle | freelance economist and a former advisor to the UK Treasury. She is a member of the UK Competition Commission and is acting Chairman of the BBC Trust, the governing body of the British Broadcasting Corporation