Humans Have a Non-Physical Nature

    We have shown that humans are physical. Are they merely physical? Are the materialists correct that everything is entirely physical, including humans? Is all knowledge therefore physical knowledge? Can everything about humans be known (in principle) in terms of matter, energy, and mathematical laws? The key insight into these questions is the knowledge argument, put into current form by Frank Jackson. This argument is motivated by the realization that human consciousness is special. Intuitively, there is something decidedly non-physical about consciousness. However, attacking the problem of consciousness directly is one of the more challenging problems in philosophy. For this reason, the knowledge argument looks at a simpler problem, that of qualia (to be defined in a moment). Because qualia themselves are difficult, we start with an even simpler problem, which is a thought experiment in which all reasonable people agree on the data, if not necessarily the interpretation.

    We shift our focus here from physics to neuroscience, but the change in focus is not as great as it first appears because neuroscience (and all other science) is understood (as a fundamental principle of science) to completely reduce to physics. Imagine Mary, who is born completely color blind. In all other ways her vision and experiences are the same as any other human. As she grows up, she becomes interested in color, and becomes the world's pre-eminent color scientist. She understands photons, rods, and cones. She understands how the brain measures the first axis of color information given the signal on the red versus the green cones and how the brain measures the second axis of color information given the signal on the blue versus the red and green cones. She learns everything that physics and neuroscience can teach about color vision. Given any visual stimulus, she can experimentally determine what neurons fire in a given brain and measure what physical color information that person or animal acquires.

    After she has learned all the physical facts about color, medical science advances to the point that her visual defect is completely corrected, so that she can now see color. Now when she sees watered grass, she says, "Ah ha! So that is what it is like to see green." When she sees a ripe tomato, she says, "Ah ha! So that is what it is like to see red." She knew all physical facts before. She has learned a new fact: what it is like to see color. Therefore, there are facts in the universe which are not physical. We call these facts phenomenal. Not coincidentally, these new facts are tied to consciousness. The conscious sensations of seeing colors, feeling pains, sensing warmth, etcetera are referred to as qualia (singular: quale). Note that neuroscience has no place for consciousness, even though we all are aware of it. This is the most important clue that neuroscience cannot tell us everything there is to know about being human.

    Let us look more deeply at the knowledge argument. What does it mean when we say that a fact is unknowable using physics? What we mean is that no matter how many textbooks Mary looks at and no matter how many conversations she has, she will not be able to learn what red looks like without experiencing it for herself. That is, what it is like to see red cannot be communicated with language. Physics is a mathematical language that describes nature, so it follows that anything that cannot be communicated with language cannot be communicated with physics.

    Let us make our description of physics as a language more precise. Extrapolating from information theory (originally developed for computer science, but now applied to many fields, including physics and neuroscience), any distribution of matter and energy can be represented as a series of bits (that is, zeros and ones), where the bits are information. In order to see how the bits change with time, it is necessary to have operations (add, shift, etcetera) which act on the bits. These operations are used to represent laws of physics. Essentially, a fact can be represented in the language of physics if, and only if, it can be represented with bits and operations. An ideal physicist can translate any measured distribution of matter and energy into bits, run these bits through the operations that represent the laws of physics, and translate the resulting bits into predictions about the probabilities that future measurements will return specified results. It follows that any field that reduces to physics (neuroscience, computer science, etcetera) can also in principle be expressed in its entirety with bits and operations. Mary could never learn what it is like to see red by looking at bits and operations. She must experience redness for herself. The knowledge argument teaches us that the epistemic power of physics, and therefore neuroscience, is limited; and it can never explain every known fact as the materialists expect it to.

    Let us push the knowledge argument a bit further. Can an entirely physical brain know phenomenal facts? No. A physical brain stores information in bits (spiking activity, individual neuronal characteristics, and neuronal connections) and processes the bits with operations (spiking neurons cause others to spike, which can cause those neurons to change their characteristics, connections, or cause them to spike). The physical world stores information in bits (distribution of matter and energy) and processes the bits with operations (the laws of physics). With the knowledge argument, we demonstrate that knowable phenomenal facts exist and that physics (or any collection of bits and operations) cannot contain them. Because the physical brain only has the functional capabilities granted it by physics, it cannot contain phenomenal facts either.

    I have focused the discussion on sensory qualia in order to make the problem of consciousness concrete. However, it is apparent that all aspects of consciousness lead to similar questions. We can ask: What is it like to add 1+1? What is it like to use language? What is it like to pursue an ideal? What is it like to love? You can imagine building arguments similar to that of the knowledge argument for each domain, though as the topic becomes more abstract the thought-experiments become less intuitive. More simply, though, it is apparent from our experience that all aspects of consciousness have a shared nature, and therefore must have a similar non-physical origin.

    It is also apparent that consciousness includes not only knowing "what it is like" but all non-physical knowledge. This includes knowing who you are or I am. It includes knowing what trees are, physics is, or how to do philosophy. It includes knowing God. A physical brain can only store and process physical information, while a non-physical mind is necessary for non-physical knowledge. The distinction between information and knowledge can be subtle in many specific cases, but there is no doubt that in general there is a distinction.

    Next, we will discuss free will. Later, we will discuss the balance between the physical and phenomenal nature of humans in more detail.

    Also realize that this argument implies that any conscious being, including God, must have a non-physical nature.

Objection 1:

    St. Thomas Aquinas says (Summa Theologica I.75.3) that sensory experience is inherently physical because it requires a body, while the intellect is inherently non-physical and does not require a body.


    The front end of seeing is indeed physical. This is the process by which light is caught by photoreceptors and transmuted into electrical-chemical impulses in the retina and brain. However, the back end is non-physical. This is the part where we become conscious of what we are seeing and "know what it is like." Note that the act of seeing does not intrinsically require the awareness of seeing, as robots can process color, but have no awareness.

    The front end of thinking is also physical. Aquinas refers to this as the cogitative power or particular reason, which includes comparison, computation, and memory. He correctly understands that its source is physical. "Wherefore it is also called the particular reason, to which medical men assign a certain particular organ, namely, the middle part of the head" (Summa Theologica I.78.4). The back end of thinking is non-physical. Aquinas refers to it as the intellect and it includes both consciousness and knowledge (Summa Theologica I.75.2). The cogitative power does not intrinsically require consciousness and knowledge, as a robot can compute, but cannot know. Present day philosophers typically refer to the intellect as the mind, and the cogitative power as the brain.

Objection 2:

    What is the status of animals?


    Many measured systems in animal brains exhibit substantial unpredictability. Most of this unpredictability we can expect to eventually explain with purely physical reasoning. On the other hand, neuroscience does not assure us that there is no hidden variable, the non-physical mind, which is influencing the physical brain. Anything we can understand completely using physics we can state to be physical. Currently, isolated electrons, rocks, computers, planets, etcetera all qualify. Simpler organisms, such as plants (which have no brain) and sea slugs (which have about 20,000 neurons), we can also reasonably expect are completely physical based on current experimental knowledge. As experimental techniques improve, the number of systems that can be completely explained with science will increase. It is also possible, in principle, that we will eventually be able to measure explicitly that a human mind cannot be explained entirely in terms of physics. For now, though, we can understand ourselves to have a non-physical nature because we have insider information (our own consciousness). We can safely assume, by analogy, that other humans are also conscious and have a phenomenal nature. For most animals, however, I lack the philosophical and scientific tools to make a clear determination.

This page was last changed on 2011/08/28