The defining characteristic that separates humans from other living beings in ancient philosophy is the concept of standing upright. Plato, a prominent figure among ancient philosophers, is famously attributed with the description of humans as bipedal and lacking feathers. While this description may seem peculiar today, Plato viewed the absence of feathers as a significant step away from animality towards humanity, acknowledging that other bipedal creatures, such as birds, exist. In Greek thought, birds occupy a unique position between humans and the divine, often associated with the gods through their use in divination. In a notable retort to Plato's definition, the Cynic philosopher Diogenes of Sinope brought a featherless chicken to Plato’s Academy, declaring, “Here is Plato’s man.”

Despite the obviousness of bipedalism as a defining feature of humanity, it was Plato who emphasized the importance of upright posture in elevating the rational mind above base instincts. He viewed the head as the "acropolis" of the body, symbolizing the pinnacle of both physical and intellectual achievement. Just as the state reflects the organization of the human body, Plato believed that an upright form embodies wisdom and nobility, concepts he extracts from ancient notions of beauty and goodness.

The tension between Plato’s rigid definitions of posture and Diogenes’ playful critique establishes a framework for future discussions about human posture, though the latter’s stance appears to have prevailed over time. Contemporary debates in the philosophy of science have revived the image of Diogenes’ chicken as an exaggerated illustration of what it means to be classified as human. Bertrand Russell, in the early 20th century, pointed out the distinction between the phrases "Socrates is a featherless biped" and "Socrates is human," highlighting how the latter encompasses a broader definition.

The discourse surrounding uprightness, encompassing moral, aesthetic, and physiological dimensions, has moved beyond the symbolism of the chicken to explore other powerful analogies. For Jewish thought, Adam's upright stance signified his superiority over the animals created by God, a belief that persisted into the Enlightenment. Johann Gottfried Herder, in his work "Ideas for the Philosophy of the History of Humanity," emphasized posture as a fundamental aspect of the “organic distinction between humans and beasts.”

Herder characterized the human form as uniquely upright, noting that this position is both natural and constant for our species. He described how the human body is anatomically adapted for this posture, with distinct features such as an elongated calf, a retracted pelvis, and broader hips. The upright stance, crowned by a head poised on a muscular neck, allows humans to engage with their environment in a way that reflects their unique nature.

While Herder acknowledged the importance of reasoning in defining humanity, he placed upright posture at the forefront, viewing it as a manifestation of divine creation. He depicted the moment of creation as God instructing Adam to “stand up straight,” illustrating the transformative power of this posture.

Herder’s ideas connect with those of John Milton, who in "Paradise Lost" articulated a vision of the first man as a rational being endowed with the gift of upright posture. This creation moment, where humanity attains both moral and physical uprightness, is echoed in biblical texts, reinforcing the notion that standing upright signifies a moral elevation.

Immanuel Kant, however, critiqued the theological implications of such views on posture. He regarded Herder’s beliefs as overly romanticized rather than empirically grounded. Kant argued that humans are inherently imperfect and must strive for self-improvement, emphasizing the importance of self-knowledge in the Enlightenment ethos.

In Kant’s “Idea for a Universal History with a Cosmopolitan Purpose,” he characterized humanity as “crooked wood,” suggesting that perfection is an unattainable ideal. He posited that human striving can guide individuals towards this ideal but acknowledged that nature permits only approximations of such perfection. A community founded on reason can cultivate the growth of “crooked wood” into a more desirable form.

Kant’s idea that human value arises from the effort to integrate oneself into society contrasts with Herder’s emphasis on upright posture as the essence of humanity. Kant challenged the primacy of uprightness in defining human nature, asserting that it was not the inherent rationality of humans that bestowed them with upright posture, but rather that their ability to walk upright led to the development of reason.

Later thinkers in the 19th century embraced Kant’s perspective, interpreting upright posture as a reflection of the vitality of life itself. Ralph Waldo Emerson, in a lecture in 1839, asserted that life is characterized by continuous movement, paralleling the need for dynamic adjustment in human posture.

The theological view of posture as the essence of human existence, as echoed by Emerson, persisted in the aesthetics of the 19th century. Hegel, while engaging with Kant’s ideas, proposed a theory of posture as central to understanding humanity not only as rational beings but as materially organized entities. Hegel argued that ceasing to act leads to a regression to a more primitive state, asserting that upright posture enables moral agency.

However, Hegel also pointed out that mere uprightness does not equate to beauty. He distinguished between static and dynamic human forms, emphasizing that true beauty emerges when posture conveys spiritual essence rather than mere physical alignment.

The contrast between idealized bodies in classical art and the living, dynamic human form is significant. Oscar Wilde highlighted this disconnect, questioning whether ancient sculptures accurately represented the people of their time. The confusion surrounding postural norms persisted well into the 20th century, with debates on the differences between ancient ideals and modern understandings of anatomy.

Ultimately, Hegel argued that human posture is defined by the will, emphasizing that the ability to stand upright is a product of conscious effort rather than divine decree. The collapse of posture signifies a return to a primitive state, reinforcing the notion that human identity is inextricably linked to the capacity for self-determination and moral action.