René Descartes maintained the view that humans do not directly perceive the world and its contents as they truly are. Rather, he postulated that one relies on incomplete information derived from senses that merely give a representative impression of reality, at best. Aristotle’s and the more classical/scholastic views of natural philosophy held that all knowledge available to human understanding came from direct sensory experience of the world, and that true knowledge could only be attained in an empirical fashion. Aristotle claimed that forms, the innate essences of the objects in the world, reflected themselves inside the mind of the human. He also believed that the senses provided a complete and clear picture of reality and its inner workings. To Aristotle, hot things were hot because heat was manifested within their current substance, as one of the states of their natural forms, and this heat manifests in the mind if one is to touch it. An example would be a fire or a star; there are many fires and many stars, but to Aristotle they all tend towards the natural states of their forms, which is to be hot. This classical scholastic view of natural philosophy said that humans perceive heat, color, taste, or any property, spatially extended or not, as these experiences actually are. Descartes objected to this view by refusing to ascribe to the idea that the senses precisely reflect the world, or even necessarily provide any resemblance of reality at all.
A major case Descartes made for his argument against what he called “naïve empiricism,” or what he saw as blindly believing the senses, was his assertion that representation does not imply resemblance. An example he gave in his essay Optics was of a blind man using a stick to find his way; simply because the man has a refined a certain sensory representation of part of his world, does not mean that he has anything close to a full perception of it. From there he argued that humans do not directly perceive the world despite our ability to gain impressions of light with our eyes, or our other senses such as touch or taste, because they do not fully account for objects’ properties and extensions, but rather only the parts of reality narrowly perceptible to these senses. Aristotle saw qualitative properties of objects, such as color, as intrinsically real and substantially existing in a given object, thus producing a direct representation of the object’s true nature in the mind. Descartes argued that colors are merely something of a “trick” the eyes use to interpret light and process the outside world, and that color was simply the brain’s attempt to understand the various “spins” of the air particles to distinguish objects with color. He used this as a criticism of the idea that our senses reflect the actual nature of the world. Descartes attempted to strip away what he saw as unreal, or at least exclusively perceived in the human mind. He narrowed his definition of true physical extension of an object down to the cornerstones of size, shape, and motion, and all else was simply qualitative and produced by the senses.
Descartes represented his ideas about perception was by showing how sensory input and motor output seemed to be mechanistic and connected through the brain to where he thought the soul’s rapport to the mind to be: the pineal gland. An example in his intentionally non-philosophical but important physiological observations in Treatise on Man that showed how the eye simply receives a reflection of the light, like the photographic art technique known as camera obscura that reflects an image through a focused, concave lens. Like the eye, this flips the object vertically and projects it onto a wall or other medium. Descartes’ analysis of the eye led him to conclude that the eye is simply a camera obscura-like device that collects impressions of light. His analysis of the sense of visual perception was that light enters the eye through the retina, which then submits signals through the brain to the pineal gland. He believed that all of the senses were ultimately linked to the pineal gland, where imprints made on it created sensory experience, allowed humans to interpret the signals, respond with logical thought, and take appropriate action.
Descartes ultimately used this mechanical view of sensory perception to disagree with the Aristotelian concept of the three types of “souls,” these being as such: “vegetative” which belonged to plants, “sensitive” which belonged to animals, and “rational” which belonged to humans. Aristotle’s view of plants was that it is in the nature of their forms to be nourished and to reproduce. Likewise, he saw animal souls as having various forms that naturally perceive, move on their own, and can be nourished and reproduce like plants. The human soul, to Aristotle, encompassed all these characteristics that animal life did, and also contained the capacity for logic and reason. These souls were what Aristotle supposed that caused things to live, move, act, and persist through the ages, by fulfilling the “natural” states of their universal forms.
Descartes’ view differed, theorizing that instead of a human soul-form pushing people toward their “natural state,” the pineal gland contained a more physical and substantive fluid-like “animal spirit” that drove the life forces of a human, and facilitated the individual soul’s control of bodily movement through the pineal’s piloting of the brain. A clarifying mind-experiment that Descartes proposed in Treatise on Man is that of statues made animate with various mechanical parts, using various input as a means to respond with resulting action by shifting around fluid “spirits,” driving the complex machineries within and causing them to be able to imitate human actions. A hole is left in the statue’s head, empty of a soul where it would normally function in a human; if a soul were to be placed in one of these statues, it would feel and act like a human. He used this to question the requirement for such a being to have a soul or mind, when these statues can be animate and not require a thinking soul of any kind to function, much like the modern perception of a robot. This goes against the Aristotelian concept of souls because none of these actions could be considered borne of soul or life, only machinery, yet these statues could imitate or even fulfill the Aristotelian requisites for life.
Descartes’ dissection and analysis of the human heart showed it to be, put simply, a pump for circulating blood. Moreover, his dissections of the body and brain made him see the human body like an organic machine controlled by the brain, very similar in nature to these robotic statues. However, Descartes posited that while these hypothetical mechanical statues could imitate human activity and reactions in given situations, they could not have the capacity for logic because of the lack of a reasoning mind/soul. They could likely eat or sleep, but not hold a meaningful conversation with you. From there, Descartes drew the line, likening these statues to animals, since he saw animals as simply creatures of reflex and instinct. However, he claimed that while these statues could be devoid of souls or minds and still function, this does not inherently exclude the possibility of a mind-like perception in animals. Rather, it implies an absence of the logical, rational soul innate to humans, which he viewed as the only true “mind.” He saw that an animal mind could result from their sensory experiences routed to their brain, such as with eyes like those of humans. Through recognizing this key division and also animals’ lack of capacity for language, Descartes felt cemented in his idea that animals, while possibly sensing and perceiving, have specific, programmed reactions to stimuli that are caused by mechanical interactions of the senses, much like machines, whereas humans perceive sensory information impressed upon the pineal gland, the seat of the soul, to make sense of the world around them with a rational mind, regardless of the accuracy of the input.
Descartes’ primary criticism of the Aristotelian and Scholastic concepts of human and animal physiology, as well as sensory perception, is that biological beings are but cleverly-constructed machines, using representative yet incomplete senses to feed the mind information to respond accordingly. Because of this disparity between reality and sensory experience, Descartes essentially argues that a priori logic and knowledge that need no physical evidence are more effective than a posteriori learning as a result of empiricism, because sensory representation cannot be correlated to resemblance of reality.