Consciousness: The greatest challenge in neuroscience

In 1974, the American philosopher Thomas Nagel, a professor at New York University and a major figure in the philosophy of mind, published what would become a landmark paper entitled What Is It Like to Be a Bat?. Behind a question that may appear deceptively simple lies a central problem in the scientific study of consciousness. Nagel raises a radical issue: how could we ever claim to know what it is like to be a bat, that is, to possess its own distinctive mode of experiencing the world, even when we have extensive knowledge of its biology, nervous system, and perceptual mechanisms?

The example is particularly illuminating because bats rely on echolocation, a sensory system that allows them to perceive their environment in a way fundamentally different from our own. Even with a precise understanding of the physical and neural mechanisms underlying echolocation, something remains irreducibly inaccessible: the lived experience itself, the what it is like. Nagel concludes that any strictly objective theory of the mind inevitably leaves out a core dimension of consciousness, namely its subjective and phenomenal character (Nagel, 1974).

The hard problem of consciousness: the ultimate frontier of brain science

This observation resonates directly with the work of David Chalmers, a philosopher and cognitive scientist who, in the 1990s, introduced the now famous concept of the hard problem of consciousness. Chalmers distinguishes between the so-called easy problems of consciousness, which are only easy in a methodological sense, and the genuinely hard problem.

The easy problems concern cognitive functions that can be objectified and measured, such as attention, memory, perception, planning, or the ability to verbally report mental states. These processes can be investigated using neuropsychological testing, brain imaging techniques, and computational models (Chalmers, 1995).

The hard problem, by contrast, raises a fundamentally different question: why and how are these physical and functional processes accompanied by a subjective, lived experience? Why is there something it is like to perceive a color, feel pain, or experience an emotion? According to Chalmers, consciousness is not, and may never be, reducible to purely quantitative parameters. It is fundamentally rooted in subjective experience, lived in a singular way by each individual, regardless of how precise our objective knowledge of the brain may become.

This irreducibility is powerfully illustrated by the famous thought experiment known as Mary’s Room, proposed by Frank Jackson. Mary is a neuroscientist who knows everything there is to know about the physical and neurobiological laws governing color vision, yet she has spent her entire life in a black-and-white environment. When she sees the color red for the first time, does she learn something new? Intuitively, the answer is yes. She gains access to the lived experience of red. This thought experiment suggests the existence of a form of phenomenal knowledge, knowledge of what it is like, which cannot be reduced to objective or propositional knowledge (Jackson, 1982).

It is within this framework that the concept of qualia emerges, referring to the qualitative aspects of conscious experience: what it is like to see red, taste sweetness, hear a melody, or feel a specific emotion. Qualia lie at the very heart of the hard problem of consciousness and largely explain its resistance to any attempt at direct measurement.

Indeed, while cognitive science and neuroscience have made remarkable progress in identifying the neural correlates of consciousness, these advances primarily address the so-called easy problems. Attention, memory, and certain forms of perception can be measured objectively and reproducibly. What remains deeply problematic is precisely the reluctance of subjective experience to be captured by standardized measurement tools.

The core enigma of consciousness

Beyond philosophical thought experiments, several theoretical models have attempted to address this challenge by drawing in novel ways on neuroscience and quantum physics. One of the most well known examples is the Orchestrated Objective Reduction model, proposed by Stuart Hameroff and Roger Penrose. According to this model, consciousness emerges from quantum processes occurring within neuronal microtubules and therefore cannot be reduced to classical synaptic interactions alone (Hameroff & Penrose, 1996).

This model has attracted substantial criticism, particularly because of the current difficulty, or even impossibility, of empirically testing it under laboratory conditions. Nevertheless, it remains intellectually compelling and opens a dialogue with certain spiritual perspectives by suggesting that human consciousness may be embedded in a broader reality than the biological body alone. From this standpoint, the idea that neuronal reactions and interactions are sufficient by themselves to generate consciousness may appear overly reductionist.

This question also lies at the heart of research in computational neuroscience and artificial intelligence. Although we are now capable of modeling systems that exhibit intelligent behavior, learning, and decision making, there is no evidence that such systems are conscious. This distinction is crucial and aligns with the work of Antonio Damasio, who proposes a graded conception of consciousness, distinguishing proto-consciousness, core consciousness, and extended consciousness (Damasio, 1999).

For Damasio, consciousness is inseparable from the body, emotions, and interaction with the environment. It depends on a body capable of generating affects, in continuity with the tradition of William James and the James-Lange theory of emotion. It is precisely this embodied, affective, and situated dimension that is strikingly absent from artificial intelligence systems, regardless of their computational performance.

Thus, the hard problem of consciousness remains a major point of tension between philosophy, neuroscience, and cognitive science. Far from being a mere methodological obstacle, it invites us to rethink the very foundations of our understanding of the mind by integrating subjectivity, the body, and the environment as constitutive dimensions of conscious experience.