This most fundamental question in philosophy is, "What exists?" This is typically framed as a debate concerning the difference between mind and matter, and how they interact.¹

 

Most of us think of mind and matter as very different things. Matter is something that we can reach out and touch — it's physical, and it has semi-stable properties, such as size, shape, color, etc. But the human mind seems to be very different. We can't reach out and grab it, and it constantly flits about, here and there, without any respect for inertia, cause and effect, or the unidirectional procession of time. Hence it seems that there are two different realities: the mental and the material, and they play by very different rules.

 

René Descartes (1596~1650)

Descartes built this difference into a metaphysical system now known as Cartesian Dualism. Furthermore, he believed that God lives in the mental realm. The proof was that thoughts can be perfect, while nothing in the material world is perfect in the same sense, and because God is perfect, we must get such thoughts from Him. For example, Descartes observed that he could inscribe a circle on a piece of paper using a compass, and from a distance, it might look like a perfect circle. Yet if he used a magnifying glass, he could always find imperfections. He concluded that he was not getting the sense of perfection from the circle itself, which meant to him that the perfection had to come from elsewhere. And that could only be from God.

 

So how do the mental and material realms interact?

 

Descartes supposed that the pineal gland (at the top of the brainstem) mediated between the body and the soul, but without saying how. Other philosophers had other ideas — some think that the mental and material realms are totally separate, and just happen to stay synchronized, while others believe that it is only the Spirit of God that can cross the boundary. Many religions have the idea that angels and/or messiahs can go back and forth, while mortals only know the physical world that they can see, and only become aware of the other side when inspired by the Holy Spirit, angels, or messiahs.

 

Galileo Galilei (1564~1642)

Descartes' insistence that we can find the truth within our own minds, irrespective of the final authority of the Catholic Church, was well-received by Europeans during the Enlightenment. But of the two realms that Descartes identified, it was the study of the material world that came to dominate. Scientists such as Galileo (1564~1642) and Newton (1642~1727) laid the foundation for a mechanistic model of the Universe, in which all things are the products of simple physical interactions between primitive components. John Dalton (1766~1844) proposed the Atomic Theory of Matter, and Dmitri Mendeleev (1834~1907) discovered the Periodic Law, making it possible to start to see how the components actually fit together to produce the observed complexities. Meanwhile, the emerging study of electromagnetism had profound biological implications. Benjamin Franklin (1706~1790) proved that lightning produces the same physical sensation as a spark from static electricity (kids, don't try this at home). Luigi Galvani (1737~1798) found that muscles could be electrically stimulated. So both the input and the output of the brain were shown to be electrical. Then Luigi Rolando (1773~1831) and Pierre Flourens (1794~1867) took the next step, applying small currents directly to brain tissue, finding that they could trigger different mental functions (e.g., recollections, emotions, involuntary vocalizations, etc.), depending on where they stuck the prod. Subsequent research in the 1900s, such as brainwave studies and PET scans, have led to the conclusion that for every subjective state, there is a corresponding neural activity pattern.

 

With all of this evidence, we can now say that Descartes was wrong about the mind playing by non-physical rules — it simply plays by a set of rules that hadn't been discovered in Descartes' day. Electrochemical impulses travel through the brain at over 100 m/s, and they can retrieve long-term memories virtually instantaneously, without respect for inertia. This is no more of a metaphysical problem than wondering how we can take a picture of an object, itself so heavy that it can only be lifted by a crane, and then hold the picture just with two fingers — that isn't a violation of Newtonian mechanics. Similarly, long-term memories are just pictures of objects, and pictures don't have the same physical characteristics as their subjects. But that doesn't mean that they aren't physical themselves. Likewise, thumbing through the pages in a history book backwards doesn't make time run in reverse, violating the laws of causality — the events described therein still happened in causal order, even if the perception of them is out of order. Hence the contention that the mind does not obey physical laws is just not correct. The mind simply obeys its own physical laws, which are different from the laws governing the objects of its perceptions.

 

The rise of materialism, especially late in the Victorian Era and continuing on into the 1900s, actually began to displace the study of the human mind. If we are all just biological machines obeying physical laws, we might not have minds at all, or if we do, they don't do anything. And if the behavior of a human being can be fully described in mechanistic terms, without the need for the machine to be conscious, and if consciousness wouldn't do anything if it did exist, it is functionally non-existent, and need not be considered in a scientific study of the human condition. At best, our subjective perception of the world is just along for the ride, passively observing the workings of the machine from the inside, but with no volition. In this sense, Cartesian Dualism persisted, but with the locus of control shifted, from God and/or the human mind as prime movers, to physical primitives obeying natural laws.

 

G. Berkeley (1685~1753)

Yet at the same time, another school of thought has maintained that the material world is actually the illusion. The cold and impersonal Industrial Revolution was too proud of itself. It created a lot of wealth, and lots of really neat gadgets, but it lost sight of what it actually means to be alive. So as a reaction against the mindlessly mechanistic ways of material things, Idealists have asserted that we are alive in spirit, if at all, and that perception is the only true reality. And scientists studying ways in which perceptions alter (or even manufacture) realities have been just as prolific as those studying the physical mechanisms inside the brain. Branching off from the idealist side of Cartesian Dualism, philosophers such as George Berkeley (1685~1753), Immanuel Kant (1725~1804), and Georg Hegel (1770~1831) laid the foundation for this movement, with scientists in the 1900s, such as Sir Arthur Eddington (1882~1944) and Sir James Jeans (1877~1946), bringing it into the mainstream.

 

So which is it, a material world, or a mental world, or both?

 

Recent collaboration between philosophers and neuroscientists has yielded another possibility, that both matter and mind are contained within the same realm, without a difference of kind between them. They are saying that all matter is mind, and all mind is matter.²