If I tell someone I am concerned with the problem of pain, they frequently ask me whether I mean physical pain or mental pain. That question expresses the dualism of our culture. If I say 'My foot hurts me', I express the dualism of my thinking. One arm of my dualism is the thinking, talking, feeling, suffering, cognitive, mental me. The other arm of my dualism is the stuff of my body, which sends a message to the mental one that it is damaged. The structure of the sentence is exactly the same as 'The dial on my dashboard says that the engine is overheating'. It all seems so transparently, intuitively obvious.
However, if we are to consider pain, we should first be quite certain of what appears obvious: that we inhabit two separate interconnected worlds, the physical and the mental. If dualism is that starting point, then the route of our exploration is set by an initial search in the machinery of the body for a sensory system which delivers messages that create sensation, after which there is a second phase of our search for the mental processing of received messages. That route has been taken for two thousand years from Aristotle to John Searle and Daniel Dennett. Pain has been used repeatedly as the simplest possible example of a physical stimulus which inevitably results in a mental response. We will not retrace this route, dropping the names of Bacon, Hume, Berkeley, Kant and Wittgenstein, who have brilliantly described their version of the journey from sensation to perception. Nor will we join my fellow emeritus academics in their obsession to greet our oncoming senility with a discussion of consciousness. Two key philosophers will be sufficient for me. Descartes wrote in 1640:

If for example fire comes near the foot, minute particles of this fire, which you know move at great velocity, have the power to set in motion the spot of skin on the foot which they touch, and by this means pulling on the delicate thread which is attached to the spot of the skin, they open up at the same instant the pore against which the delicate thread ends, just as by pulling on one end of a rope one makes to strike at the same instant a bell which hangs at the end.

This is precisely the same formal structure of a sensory signalling system which many accept today. Descartes was well aware that it was necessary to provide a transition zone between the bell and the next stage. He therefore proposed a further mechanism where the threads came together to reach the seat of imagination and common sensation. Here 'external objects are able to impress the mind'. He uses the word 'esprit', meaning mind, soul or spirit.
In contemporary terms, Sir John Eccles, Nobel laureate neuroscientist, and Sir Karl Popper, philosopher, describe this area where the threads come together as the 'liaison area'. They state: 'The unity of conscious experience is provided by the self conscious mind and not by the neural machinery of the liaison areas of the cerebral hemispheres'. They proceed: The self conscious mind can scan the activity of each module of the liaison brain or at least those modules tuned to its present interest . . . The self conscious mind has the function of integrating its selections from the immense patterned input it receives from the liaison brain in order to build its experiences from moment to moment.

Here contemporary scientists bring Descartes up-to-date and locate the frontier between body and mind, sensation and perception in the cerebral cortex.
Descartes was in trouble in his own day. A marquise challenged him to explain how his scheme permitted the phenomenon that a man with an amputated leg sensed in every detail his missing leg. This persistent observation by amputees, or 'phantom limb', is not vague. An arm is felt in every detail with the hand and each finger. Descartes replied:

Descartes hides behind the authority of his own subjective introspection. He expresses complete conviction. How does he come by this certainty? Are there any facts? There are no facts. It is only a manner of thinking that seemed compellingly obvious and apparent to Descartes and to a very large number of people in his and our culture.
In order to separate the mind from the body, Descartes had to invent communication channels, the sensory nerves, by which external objects and internal events could 'impress' the mind. He knew nothing of the details so he had to invent them by guesswork. However, the general idea seems plausible if a mind-body separation is accepted. The sensory communication channels could be described in mechanical terms as linked chains and rods. Beyond this mechanism, the mind operated on fundamentally different principles which Descartes did not describe.
The alternative to dualism is monism, which proposes that mental processes are inherent outcomes of bodily processes. The power of the Roman Catholic Church to declare certain ways of thought to be heretical and thereby to dominate the course of philosophy may have declined. However, the growing exploration of monism as an alternative way for the analysis of thinking led to the proponents being accused of a new form of heresy. Monists are labelled as simple-minded mechanists who deny any of the engaging properties of humans and who see the entire human being as nothing but a mass of interlocked gear wheels. Dualists of the Eccles and Popper variety describe a first-stage deterministic body machinery, rigidly performing its ordained tasks, which is observed by a conscious mind full of ideas, feelings and emotions. By assigning all of the glories of humans to an unobservable mind, they trivialize the body's machinery. The alternative is that the body is an integrated whole from whose properties emerge intellectually separable components.
Bertrand Russell was certainly a candidate for the cleverest man of our age. In his History of Western Philosophy he proposes a strict form of dualism but for reasons quite different from Descartes. He begins at the top with rational thinking, which was the greatest challenge for him. His biography shows him rapidly bored with those colleagues, children and discarded girlfriends incapable of rational thought as defined by him. His Principia Mathematica, written with A. N. Whitehead, was the achievement of many years of struggle in creating a symbolic logic in which the rules of consistent relationships were laid out. The mind is evidently capable of manipulating abstract symbols and of placing them in logical categories. However, the ultimate origin of these internal mental digestions depended on observation of the nature of the external world at their beginning, and the verification of their rationality depending on checking with the outside world at their end. He was not concerned with hallucinations or fairy stories. The whole rational process begins with data (that which is given) from the outside world.
There is a political analogy very similar to this process. A dictator seeks to establish thought control by seizing all means of communication. When a civil war breaks out, the first target is the television network, followed by blocking the international telephone links and the expulsion of foreign correspondents. In this way, the dictator intends to feed the citizens only with the information he wants them to learn. A wise dictator, of whom there are fortunately few, takes great care to ensure that he is the one person in the country who has full access to all available information. All too often, the leader neglects this elementary precaution and surrounds himself with sycophants who tell him what he wants to hear and spin-doctors who so skilfully manipulate the news that everyone is confused.
Russell understood that censorship of input would be a disaster and anathema to rational thinking. For this reason, he was more than content to support the Cartesian idea of a mechanical information centre in which all of the available information detected by the sense organs would be on constant uncensored display. He wished to assign to the mind the role of supreme commander fed by loyal, efficient, unquestioning messenger boys who would update him on every available bit of information, important or unimportant. This information service is entirely passive, whereas the mind is selective. Russell does not consider the role of exploration, where the organism actively seeks information. It is true that exploration has an inherent weakness because it implies a decision by the mind that one piece of information is more useful than another. However, as we shall see, every aspect of our conscious sensation suggests that it is not fed by a passive machine but includes brain activity, which has directed attention, and ordered exploration, which has selected part of the sensory input and has amplified information about details. Unlike Russell's ideal of a passive input analysed by an active brain, there are signs that brain activity controls the input. This does not mean that the entire outside world is a hallucination, but it does mean that our senses include active participation of mind and body.
If we define the mind as those processes within us about which we can give verbal reports, we should begin by examining processes about which we have no cognitive knowledge.