Ignorance-Knowledge: 125

This formula is no complex argument, but an immediate intuition. According to Descartes, the proof of "I am" does not depend on any external world; the very process of thinking is that proof. This thinking being he called the "thinking substance" (res cogitans), as opposed to "extended substance" (res extensa)—the material world or body. From this dualism of mind (consciousness) and matter (substance) was born mind-body dualism, which remained at the center of philosophical, psychological, and neuroscientific discourse for centuries to come.

This "cogito" or self-consciousness of thought became for Descartes the first principle of all knowledge. The senses, experience, even the existence of the external world could be questioned, but the presence of one's own conscious thought cannot be denied. Here the soul becomes the foundation of knowledge; knowledge no longer depends on external objects but rests upon the presence of consciousness. Thus he established rationalist epistemology—where reason and self-awareness are the only reliable sources of knowledge.

Yet "Cogito, ergo sum" is not merely an argument; it is an existential realization. It declares: "The truth of my existence lies not in external proof but in the continuity of my thought." However deep the doubt may go, the reality of the doubting "I" remains. Hence this maxim teaches us—doubt is no weakness; rather, certainty is born from within that very doubt.

This insight revolutionized the history of modern philosophy. Husserl shaped this self-awareness of consciousness in his phenomenology into a project of "pure phenomenological consciousness"; Heidegger in his Being and Time declared that humans do not merely think—they exist (Dasein), and knowledge lies embedded within that experience of existence. Sartre transformed this concept into the language of existentialism, where "consciousness" means such a self-opening that continually creates its own existence.

René Descartes's two fundamental concepts in philosophy—"res cogitans" and "res extensa"—established a profound dualism for understanding the nature of human existence and reality. "Res" is a Latin word meaning "thing" or "entity"; "cogitans" means "thinking," "extensa" means "extended." Thus the literal meaning of these two concepts becomes "thinking entity" and "extended entity."

Starting from his "Cogito, ergo sum"—"I think, therefore I am"—formula, Descartes found a fundamental division within human existence itself. He said that what I know with absolute certainty is that I am a thinking being—res cogitans. This thinking being or mind is self-aware, immaterial, unextended; and perceives itself through introspection. It can think, doubt, will, feel; but it has no volume, weight, or spatial location. The mind is thus a "placeless entity"—unlike the body, yet experiencing through it.

On the other hand, res extensa is the material world—extended, spatial, measurable, divisible, and governed by mechanical laws. This is the realm of inert reality—matter, body, mountains, planets, everything belongs to this category. The qualities of this world are quantity (quantitas), form, location, and motion—that is, everything determined by space-time-law. Descartes said that just as the mind's thought is internal, matter's motion is external; the mind knows, matter extends.

In trying to explain the relationship between these two fundamentally different entities, Descartes found himself in a deep philosophical crisis—which later became known in history as the "mind-body problem." When he sought to explain human beings, he said—humans are constituted by the union of two entities. On one side, the thinking substance (res cogitans)—which is the soul, mind, or source of consciousness; on the other side, extended substance (res extensa)—determined by body, matter, space, and motion. But these two are of entirely different natures. Mind is immaterial, unextended, grasped through feeling and thought; body is material, measurable, spatial, and works mechanically. So Descartes faced the question—how are these two opposite entities connected? How can a thought move the body, or how does a bodily injury create mental pain?

Seeking answers to this question, Descartes proposed an unprecedented notion—the seat of the soul. According to him, a small gland located deep within the brain, the pineal gland, is that special place where soul and body connect. In his work The Passions of the Soul (1649), he wrote: "My opinion is that this pineal gland is the principal seat of the soul and the place where all our thoughts are formed." Descartes had observed that all other parts of the brain are paired (two hemispheres), but this small gland is single (unpaired); since consciousness is a unified experience—we think with one mind at a time—he wanted to find a unified center.

Descartes's explanation was inspired by the mechanical age. He believed that the body's nerves were like tiny fluid-conducting tubes; messages from the senses reach this central gland through that fluid. There the soul receives those messages, responds through will or thought, and that again returns to the body through nerve-fluid motion, creating movement. Thus body-mind connection was conceived as a dynamic process. The pineal gland thus became in Descartes's philosophy the connection point of human consciousness—where soul meets body, where thought takes bodily form.

However, this explanation quickly faced criticism. First, if mind and body are naturally separate from each other—one immaterial, the other material—how can their connection occur through a material organ? Second, it was later discovered that the pineal gland's actual function is the secretion of melatonin hormone, which regulates sleep-wake cycles—it has no direct relationship with consciousness or thought. Modern neuroscience no longer considers this gland the seat of the soul; but Descartes's endeavor remains a milestone in the history of understanding mind-body relationships.

Philosophically, the concept of "seat of the soul" carries a symbolic truth—it signifies that humans are neither merely body nor merely thought; their consciousness and body are interconnected. Though Descartes confined this relationship to a specific physical location, his real purpose was to explain the connection between these two—how thought creates action in the material world, and how material changes awaken mental experience.

Descartes's division of body and mind had far-reaching effects on philosophy, where the body was seen as merely a machine and the mind as an incorporeal entity. But later philosophers, especially Husserl and Heidegger, challenged this rigid division. They argued that humans are never merely 'res cogitans' (thinking substance) or 'res extensa' (extended substance); rather, they are always 'being-in-the-world.' This concept means that our existence is never isolated but always intimately connected to the world and constituted through experience.

Through his phenomenology, Husserl shows that consciousness is not merely an abstract process; it is always 'intentional' toward something. That is, consciousness is always directed toward some object, and this object finds its meaning within the world. On the other hand, Heidegger introduces the concept of 'Dasein,' which signifies human existence—a being that exists not merely materially but is conscious of the meaning and possibilities of its existence. According to Heidegger, Dasein is always 'thrown into the world' within a specific historical and social context and forms its relationships with that world.

These philosophers believe that consciousness and body reflect each other; they are no mechanical connection but a unified living experience. The body is not merely the container of the soul but the medium of the soul's expression. Consciousness perceives the world through the body, and the body expresses itself through consciousness. This relationship is so profound that separating one from the other is impossible.

When Martin Heidegger contemplates human existence in his magnum opus Sein und Zeit (1927)—"Being and Time"—he employs a distinctive term: "Dasein." This is a German word; Da means "there," and Sein means "existence" or "being"—that is, "there-existence" or "being-in-the-world." Within this word lies Heidegger's entire philosophical intention—humans are not abstract thinking souls; they "are there," in this world, within time and relationships—a real, finite, situated existence.

Where Descartes's "Cogito, ergo sum" ("I think, therefore I am") defined humans as thinking substance (res cogitans), Heidegger overturns that notion. He says humans cannot be known merely through "thinking"; humans first exist, then think. They are not isolated "minds" but embedded beings—always "being" within some context—in some world, in some web of relationships, in some flow of time. This being-in-the-world or In-der-Welt-sein is the fundamental structure of Dasein.

For Heidegger, "existence" (Sein) is not a static or universal concept; it constantly reveals itself (aletheia) through our everyday experience. When humans do something, think, love, fear, hope—through all these ordinary states existence manifests itself. Therefore he says philosophy's fundamental question should be "What is Being?"—but the answer to this question comes not from some other theory but from humans themselves, because humans are the being that questions existence. For this reason he called humans—"the guardian of the meaning of Being."

Aletheia—the word comes from ancient Greek origins, a- (meaning "not") and lēthē (meaning "forgetting" or "being covered")—in the combination of these two parts. Its literal meaning becomes "un-covering," or "unconcealment"—that is, the revelation of what was hidden. In the poetry of the ancient Greek philosopher Parmenides, this word first enters philosophy's domain, where he said—truth (aletheia) means that state of knowledge where what was hidden has been revealed.

But Martin Heidegger revives this word with an entirely new philosophical meaning. According to him, truth is not the matching of propositions or descriptions with reality, as modern epistemology claims—rather, truth means the unconcealment of Being. That is, in the way things present themselves before us, they reveal their existence while also keeping themselves partially concealed. This play of unconcealment and concealment is called aletheia.