Neuroscientist; Collège de France, Paris; Author, Consciousness and the Brain
The definition of life and consciousness?

Some scientific questions cannot be resolved, but rather are dissolved, and vanish once we begin to better understand their terms.

This is often the case for "definitional questions". For instance, what is the definition of life? Can we trace a sharp boundary between what is living and what is not living? Is a virus living? Is the entire earth a living organism? It seems that our brain predisposes us to ask questions that require a yes or no answer. Moreover, as scientists, we'd like to keep our mental categories straight and, therefore, we would like to have neat and tidy definitions of the terms we use. However, especially in the biological sciences, the objects of reality do not conform nicely to our categorical expectations. As we delve into research, we begin to realize that what we naively conceived of as a essential category is, in fact, a cluster of loosely bound properties that each need to be considered in turn (in the case of life: metabolism, reproduction, autonomy, homeostasy, etc..). Thus, what was initially considered as a simple question, requiring a straightforward answer, becomes a complex issue or even a whole domain of research. We begin to realize that there is no single answer, but many different answers depending on how one frames the terms of the question. And eventually, the question is simply dropped. It is not longer relevant.

I strongly suspect that one of today's hottest scientific question,s the definition of consciousness, is of this kind. Some scientists seem to believe that what we call consciousness is an essence of reality, a single coherent phenomenon that can be reduced to a single level such as a quantum property of microtubules. Another possibility, however, that consciousness is a cluster of properties that, most of the time, cohere together in awake adult humans. A minimal list probably includes the ability to attend to sensory inputs or internal thoughts, to make them available broadly to multiple cerebral systems, to store them in working memory and in episodic memory, to manipulate them mentally, to act intentionally based on them, and in particular to report them verbally. As we explore the issue empirically, we begin to find many situations (such as visual masking or specific brain lesions) in which those properties break down. The neat question "what is consciousness" dissolves into a myriad of more precise and more fruitful research avenues.

Any biological theory of consciousness, which assumes that consciousness has evolved, implies that "having consciousness" is not an all-or-none property. The biological substrates of consciousness in human adults are probably also present, but only in partial form, in other species, in young children or brain-lesioned patients. It is therefore a partially arbitrary question whether we want to extend the use of the term "consciousness" to them. For instance, several mammals, and even very young human children, show intentional behavior, partially reportable mental states, some working memory ability — but perhaps no theory of mind, and more "encapsulated" mental processes that cannot be reported verbally or even non-verbally. Do they have consciousness, then? My bet is that once a detailed cognitive and neural theory of the various aspects of consciousness is available, the vacuity of this question will become obvious.

STANISLAS DEHAENE, researcher at the Institut National de la Santé, studies cognitive neuropsychology of language and number processing in the human brain; author of The Number Sense: How Mathematical Knowledge Is Embedded In Our Brains.