We have recently looked at the interactions between human and non-human animals throughout history, and what role they may have played in defining ourselves as we are today. The conclusion we reached is that there has been no rational deliberation (but rather contingent contact by means, for example, of mutualistic coevolution) to kickstart aforementioned interplay, which historically developed into a progressively wider and more complex system of interactions. The process itself may have had, at least in theory, a role to play in the development of a wide range of abilities that have made Homo Sapiens capable of grasping some aspects of the intentions of others (be it another human being or a non-human animal).
Such an approach to the interactions between human and non-human animals did not, of course, come out of thin air but it is rather the result of a process rooted in history that has been made possible by the publication of On the origin of species (1859) by Charles Darwin. How the discussion about such topic evolved before such point of no return is what we will (albeit in a necessarily partial and incomplete way) try to address here.
Philosophic discussion has been adamant in excluding animals (non-human animals, that is) from the moral sphere at least up to the 20th Century, but the history of philosophy is not a path of progressive proximity to their inclusion within an expanding circle of moral considerations; on the contrary, we can find example of a very deep divide between humans and non-human animals even at a relatively advanced and recent stage in the history of western philosophy.
Descartes (1596-1650) in an emblematic case of exclusion of non-human animals from the field of moral consideration; his perspective demanded discontinuity between them and humans, founded on the assumption that the former are no more than matter organized in complex mechanisms but lacking consciousness and conscience granted to humankind by the presence of a soul. Non-human animals would be, in the best case scenario, extremely sophisticated contraptions capable of responding to external stimuli (although a completely “automated” and mechanical response): Descartes takes the absence of language in non-human animals as evidence of such a state of things. In his opinion such lack testifies the absence of reason; more or less complex behaviors on their behalf, such as reactions to environmental threats, would just be the result of infinitely complex but (theoretically, at least) comprehensible mechanisms not unlike those of a clockwork artifact. Language and the capacity for symbolic elaboration, on the other hand, would testify humankind has a mental plasticity and rational capacity that goes beyond a mechanical response to stimuli. Such state of things dictates, in Descartes’ perspective, that animals cannot experience sensations of pain and pleasure, since the very possibility of experiencing anything demands the presence of rational capacities (and since non-human animals lack the latter, they also lack the former). To reiterate, non-human animals would be, according to Descartes, organized yet inanimate matter akin to complex automata, with internal mechanisms (albeit extremely sophisticated, far more than what human ingenuity may reproduce) bind them to limited responses to external stimuli.
We have spoken about how the history of philosophy in regards to animals (non-human animals, that is) has not moved linearly or aiming at continuous progress. Descartes is an example of how non-human animals were completely cut off from the moral discourse, while Aristotle (384/384 b.C.-322 b.C.) may prove a less radical character to put things in perspective: non-human animals would be excluded from the sphere of moral consideration, yes, but there would be no real discontinuity between them and humankind. Humans are considered time and time again as social animals, and as such it is possible to assume Aristotle recognized at least some common ground between the realities: human beings are not the only animals to exist in groups, and the very same trait is shared by many other species. What makes humankind different seems to be the capacity to distinguish good from evil and right from wrong: this exclusively human trait, Aristotle states, is the foundation of the exclusion of non-human animals from the field of morality. Where Descartes denies the idea of animals being capable to perceive even the most basic sensations (such as pain and pleasure), Aristotle accepts that those perceptions may well be common between animals human and non-human.
The turning point in favor of the idea of continuity between human and non-human animals will come with Charles Darwin (1809-1882), and subsequent elaborations of his works will open up the scenario of the expanding circle we already tackled elsewhere. What interests us right now is the push that moved Darwin, that is to say the search for a justification for the variety of living species and its origins (topic that admittedly was, back in the day, a somewhat mainstream and greatly discussed). Despite the critics aimed at the idea of Fixism (that is to say, the concept that animal species have been created functionally identical to how they are observed currently) were many even at the time thanks to the unearthing of fossils, it remained to be understood how the transformation of species into one another occurred and what kind of mechanisms make it work. After gathering a great deal of proof both empirical and experimental (such as studies on anatomy or the observation of those mechanisms of selection exploited by breeders in his time) Darwin formulated the idea that the birth of some new species happens after an incremental build-up of variations and modifications that make carrier organisms more inclined to pass them on to their offspring (favoring, for one, the survival of the potential parents). The point making such a theory truly revolutionary is that, following the rise of each species back in time, it would be possible to reach a common ancestor of all living species.
The very last bastions supporting the idea of discontinuity between human and non-human animals fall, following this epiphany: differences between species (and between Homo Sapiens and the rest of the animal kingdom in particular) are not rooted in some superior substance or another but are rather the result of historical processes, producing the progressively more varied forms that we know today.