In this essay, I will discuss the following four points:
Those natural scientists and their followers who do not consider scientific knowledge to be limited to the realm of experiences or appearances but who consider them as objective, absolute truth, will, like philosophical materialists, view human consciousness as a product of matter, i. e. as a product of the physical organ "brain". After having shared this view myself for a long time, several ideas made me doubt it.
Until the end of the 19. century, both the philosophical and the physical concept of matter had been identical. At that time, also in natural science, atoms were thought to be the eternal and indestructible basic substance of the world. Today it is assumed by scientists that atoms are capable of dissolving into energy, i. e. motion, and thereby of disappearing. But if matter is capable of dissolving into motion then it cannot be the basic substance of the world since there seems to be something else that is in motion. Motion without something which is moving, is inconceivable to me.
This scientific theory may as well be wrong. Nevertheless it is worth noting that philosophical materialism today appears dubious also in the light of natural science. This is the more interesting since philosophical materialists generally claim to be, unlike philosophical idealists and agnostics, in accordance with natural science. 
According to the present scientific conception of the world, matter has not been in existence for ever, but was brought about in the course of the big bang. Consequently, matter has developed from simple into increasingly complex structures in the course of subatomical, chemical and biological evolution. Various elementary particles constituted various atoms, which in turn constituted molecules of millionfold variety, out of which already very complex organisms, cells, emerged and developed into cell formations. Within such cell formations, individual cells specialized in specific tasks, and nerve cells developed which then formed clusters of nerve cells. Out of these clusters, through numerous intermediate stages, the most complex of all known structures of matter developed, namely, the human brain. This was - by the assertion of many scientists und philosophical materialists - when mind and consciousness came first into existence, of which more primitive stages can be found in animals.
Now, I ask myself: why is matter doing all this? Is evolutionary pressure, as claimed by scientists sufficient to explain the development of these complex structures? If matter originally consisted of nothing else than small particles or clusters of energy with a limited number of basic physical properties, could it ever produce such complexity? Underlying this development, there seems to be a law of progress from simple to more complex structures. But doesn't follow from this that there must be some other force within matter itself or outside it which drives it to follow that law? 
However, matter doesn't just produce increasing complexity, but according to philosophi-cal materialists and many scientists from a certain degree of complexity of its structure, matter produces something entirely new, something that hadn't existed before, namely mind, consciousness. Some dumpling made of matter - and that's what the human body is from the scientific viewpoint -, a very complex conglomeration of dead, unconscious matter becomes conscious of its own existence. 
I can conceive of the possibility of intellectual achievement on the basis of a materialistic concept of the world - after all, electronic brains are also capable of executing intellectual operations -, but man isn't just capable of calculating, reading, writing, composing music, constructing engines, etc, but he is aware of it, as well. He is a knowing being, consciously experiencing joy and suffering.
If it was true that my brain, i. e. matter, produced my mind (consciousness), wouldn't then matter even in its most simple forms be much more than small particles or clusters of energy with specific physical properties? Wouldn't the capacity to bring about consciousness in more complex forms have to be founded and present in those most elementary particles? Wouldn't that mean that matter is much more than scientists recognize in it or much more than scientists are ever capable of recognizing, with their specific methods? I think that the essence of consciousness can neither be grasped nor be explained by scientific methods.
Years ago I saw a science-fiction movie in which a submarine and its crew were made that small that they could be injected into a human blood vessel. Thus they travelled to the person's brain where the crew destroyed a blood clot. Let's imagine we were able to travel through the human brain in such a way.  What would we see there? An immensely complex system of linked nerve cells; nerve fibres along which we would see electrons move at an extremely high speed; a huge biological computer from inside. But, would we see anything like "a mind", "a consciousness"? Would we be able to recognize in any way that this brain belonged to a human being, conscious of his own existence?
Consciousness, unlike matter, is invisible, it can neither be touched, nor weighed, nor measured, it cannot be put into test-tubes, it is not accessible to positivistic science, but nevertheless it is there! It is immediate and beyond doubt to the individual - at least to me, anyway. And it is essentially distinct from matter. I may put a brain into a test-tube, but I cannot do so with human consciousness. The pain I suffer is more than the fact that electrones are flowing via nerve fibres to my brain. The joy I feel is more than the process of hormons being poured into a blood vessel by some glands.
From the scientific point of view, all thoughts, feelings and perceptions have a physiological basis. As long as this assumption is the ground for practical acts, such as the curing of ill people, I find this completely right. But this does not change the fact that my conscious experience of my thoughts, feelings and perceptions is not identical with that physiological basis.
Is it possible then, that this essentially distinct consciousness is a product of matter? First I had not put this sentence as a question but as a statement, namely that this is impossible. But then I became aware that I had simply reproduced the second Cartesian proof of the existence of God: according to Descartes, the perfect cannot be the product of something less perfect.  This touches upon the issue of creativity. Is it possible that something or someone - e. g. some piece of matter, a human being, an assumed God - creates something more complex than itself or at least essentially different from itself? I cannot imagine this. Which does not mean that it might not be possible after all. And if this was possible, then also matter could produce the essentially distinct entity of mind. Whereas, if it was not possible, then nothing essentially new could be created. Then only such things could be created which are somehow already in existence. The existence and non-existence of a thing or a fact would be two contradictory and yet simultaneously existent realities. Here we get to Plato and his ideas. And here we also get to a dialectic view of existence.
If it was true that from a certain degree of complexitiy of its structure, through organizing itself into a system capable of saving and processing informations, matter could produce mind, consciousness and subjectivity, wouldn't then also some electronic brain with as much storage capacity and networking capacity as the human brain possesses, be capable of developing self-consciousness? But if this isn't possible - to me, this is an unresolved question - then something else than just the biological computer is required to constitute the consciouness of man. 
Once I happened to hit my finger with a hammer. When the pain had begun to subside so that I could think again, I noticed that my finger was not just aching but the knowledge or awareness of the pain was also in this finger and not in my head. In this situation I was hit by the realization that, in my immediate experience, my consciousness is not situa-ted inside my head at all - but it is everywhere! Wherever I feel or perceive something. When I'm looking at the screen of my computer, I have the immediate experience that the knowledge of the existence of the computer is at the place where the computer is, and not inside my head. If my consciousness was produced inside my head, would I then be able to experience consciousness outside my head? And if the world as I am consciously perceiving it, was created only inside my head (as Kant and modern science say), how does it get out of my head again? Since in my immediate experience, the world is not inside my head but it is around me. 
What was the probability at any one time in the past that from the incomprehensible plentitude of elementary particles and the inexhaustible number of their potential variations - "inexhaustible" seems to me like an understatement, but I don't know of a more suitable term - precisely my body and the very brain I call my own would emerge? The probability was zero! And this was so to such an extent that I'd nearly talk of an impossibility here. The probability of me hitting the jackpot every saturday for the rest of my life is much higher than the probability for my brain ever to have come into existence. Once you accept several presuppositions (see footnote 14, point 5), the theory of evolution offers quite plausible explanations why increasingly complex forms of life developed, and thereby why man came into being. However, the theory of evolution may be able to explain a certain structures of matter such as the human body develops. But the fact that the very specific cluster of matter which formed my body and my brain developed remains coincidential. In his life, my father probably produced some trillions of sperms. From these there emerged four children, one of which coincidentially (?) happened to be myself. The situation was similar with respect to my grandfathers, greatgrandfathers, etc. What would have happened if they had met another woman? What would have happened if the sperm out of which I once developed and which in the course of this one ejaculation had to compete with about another 300.000.000 sperms had merged with a different egg cell? Would only half of me exist today?
From the big bang to the emergence of my body there are many more such coincidences without which my body would never have come into existence. For example, the existence of a planet "earth" not too big and not too small, not too close and not too far away from a star of the size of our sun. Nor would I and some other hundreds of millions of people under fifty exist if had not been for the second world war. (All events had happened in a different course of time so that other people would have been begotten.) If my consciousness, my mind and my subjectivity were the product of my brain and if this brain had developed according to today's fundamental views of natural science, than I am the product of some co(s)mic coincidence of highest order. According to the theory of probabilities I could not possibly exist. But I do exist! That's beyond doubt! 
As far as the issue of body and mind - you may also call it the issue of matter versus consciousness - is concerned, many various explanations have been given in the course of the history of philosophy, and I don't believe that I will solve the problem. Based on my rather sceptic point of view I can only say that, when I decide to use my reason, the following three explanations seem to me worth considering:
The first possibility: matter has no existence in its own, but it is a product, or rather an idea, a fantasy of mind. Usually, once a product has been produced, it exists also independent from its producer. But that's not the way I mean it here. I do not believe that some God has created the material world from nothingness, but that matter only exists in the form of an idea of mind. 
Such a view does not necessarily lead to subjective idealism and solipsism. I believe that solipsism, taken to its consequences, leads to monism of the consciousness and therefore to a mode of idealism which abolishes ("aufheben") the contradiction between subjective and objective idealism.
In my dreams I also perceive people around me. But since from a sceptic viewpoint I cannot judge with certainty whether the dream is just a dream whereas reality is actually reality, I cannot only assume that the people in my consciousness while I'm awake are products of my mind but I could as well assume that people in my dreams are subjects existing independently of myself.
The predominant school of psychological thought holds that people in my dreams are split off parts of my own subconsciousness. But the people in my consciousness while I'm awake might as well be split off parts of my subconsciousness.
Perhaps it is the case that all people in the world of my consciousness while I'm awake are indeed independent, self-aware subjects, who have a consciousness of themselves and the world, but that all these individual conscious entities are split off parts of some all-embracing world-consciousness. In this case, the separation into singular, individual representations of consciousness would be partial and transitional, or just an illusion, or one reality which is juxtaposed to another reality contradicting the former. Then the question whether there is just one consciousness or whether there are numerous conscious entities would be just a matter of the chosen point of view. Very simply put, we would be dealing with a world-consciousness ("Weltgeist") which has somehow become schizophrenic. Thus we would also get an excellent explanation for the existence of ignorance and cruelty in the world. 
Furthermore, it would explain why, in my immediate experience, consciousness is everywhere.
If we assume that such a world-consciousness exists we will have to concede that it must be a much more complicated being than we can grasp with our present intellectual faculties, something which we can only vaguely imagine when using the term "spirit". Of course this being would have nothing to do with the God of Christianity. 
Such a monistic concept of the world with the spirit as primary element does not have to tackle the problem as to how it is possible for matter and mind to influence one another if they are, as in Descartes' view, two entirely distinct substances.
The second possibility: Matter, and therefore also my body and my brain, exists independently of my mind, but it does not produce consciousness. In this view, the consciousness of a person, which I might as well call his "soul", was simply added to the body, they were temporarily combined. In this case I'd imagine that concrete intellectual performance is possible only on the basis of the existence of the brain and sense organs and would disappear with the body's death. But consciousness itself would remain. Since consciousness entails content, the soul independent of the body would be consciousness plus x. In this case, my body and the concrete circumstances of my life could have come into existence by coincidence. Something simply happened to emerge from the tremendous number of possibilities. However, in this case this would not apply to the soul.
But if I assumed that the soul was created by some God at his own discretion, the fact that I have come into existence would still appear unlikely. Why just my soul? In the light of this explanation there also remains the problem as to how it is possible for matter and mind to influence one another if they are two distinct substances. 
The third possibility: Matter is much more than natural science knows of it. Even in its most elementary forms, matter is also mind, and includes pre-stages of consciousness.  Only because of that matter is capable of producing human consciousness in the shape of the human brain. But in this case, too, there remains the accidential nature of my existence.
This theory does not seem very plausible to me. However, it appears more plausible to me than a materialistic concept of the world. 
I wrote this text in 1987 as a part of a longer paper, "My Philosophy", in which I described how my views had changed from extremely scepticism to non-dogmatic pantheism.
("My Philosophy" is only available in German.) The text has been revised for this internet-publication. Translated from German into English in April, 1999. (Many thanks to Elena for helping me!)
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N. 1: When Marx and Engels laid the foundation for their concept of dialectic materialism in the 19. century, their use of the term "matter" was still identical with the concept of matter in natural science. With the exception of their concept of dialectics, their materialism mainly drew on the ideas of Leukippos and Demokritos who were the founders of the atom theory in ancient Greece. Confronted with new scientific findings at the begining of the 20. century, Lenin tried to save materialism with the help of a new definition of the term "matter". According to Lenin, anything which is not human consciousness is to be considered as matter, including electromagnetic fields, radiation, but also laws and anything else which is still to be discovered in the future. Thus Lenin wanted to make sure that this definition could never be outdated. But his definition is that broad that it does not explain anything anymore! This definition in fact amounted to the sell-out of materialism. If, per definition, anything which is not human consciousness is to be considered as matter, then God or any other spiritual cause of the world simply is considered to be matter, too. Back
N. 2: What kind of existence do the laws according to which matter moves and combines, these are the laws of nature, have? They exist in the form in which matter moves, but they are not matter themselves. Thus we encounter something immaterial, long before human consciousness appears. Back
N. 3: The terms "spirit", "mind" and "consciousness" are not easy to distinguish in a philosophical text. In German there are for "spirit" and "mind" only one word: "Geist". If you are translating a philosophical text from German into English, you cannot always be sure whether you have the right translation in the specific context. Especially when English is not your native language.
In his book "Wir sind nicht nur von dieser Welt" ("We are not only from this world"), Hoimar von Ditfurth, a former famous writer and TV moderator for science in Germany (died a few years ago), came to the conclusion that consciousness cannot be explained by reference to evolutionary pressure since it is not necessary for the functioning of the human body. Anything, people are capable of, they may as well perform without being conscious of it. Much of what happens in our body is unconscious anyway. (See Ditfurth, Hoimar von, Wir sind nicht nur von dieser Welt, Hoffman und Campe Verlag, Hamburg 1981, pp. 262 - 275). Furthermore it is questionable whether the development of a brain capable of higher mathematics can be explained by evolutionary pressure. This does not make sense to me. Back
N. 4: Those who know something about the history of philosophy, will remember Leibniz' parable of the mill. Back
N. 5: Descartes, Rene, Meditations on the First Philosophy, Latin and German, Philipp Reclam Jun. Stuttgart 1986, pp.111 Back
N. 6: It has been debated for some time, whether a computer will ever be capable of thinking like a human being. If man is primarily a physio-chemical mechanism, then I do not see any principle obstacles why it could not be completely or partially artificially imitated. A much more interesting question is whether a computer will ever be a subject capable of self-consciousness. Back
N. 7: When closing my eyes and imagining something, the more vivid I manage to produce the image the more it seems as if it stepped out of my mind. Then it is not in my mind anymore but in front of me. (The german term "sich etwas vorstellen", to imagine, can be taken literaly here: to put something in front of oneself). This can easily be tested by everyone: shut your eyes and try to revoke a particular image, for example of some landscape, of a person or of something else. And then give attention to your immediate experience. One does not experience the image as if it was in one's mind. One experien-ces it in front of oneself. Which conclusions one draws from these experiences, is yet another question. But I find the experience as such worth noting. Apparently, the same things happens when dreaming or having hallucinations.
The philosophers Husserl and Reininger, although in different ways, drew the conclusion that this proves the existence of things independent of ourselves. But if this was an irrefutable proof it would entail that the things in my dreams and in a person's hallucinations were independent of me, too. Back
N. 8: It is only a short step from the assumption that one has not come into existence merely by coincidence to the view that one's present situation in life is not coincidential either. Perhaps the basic conditions of our lives were defined even before we were born. This suggests the idea of an ethical causality, as in the Indian teachings of the Karma. But this is mere speculation. One should not make the mistake to take such a speculation - or to be more moderate such an assumption - for an irrevocable truth. Back
N. 9: To avoid misunderstandings it must be emphasized that in the light of this view the material world and the laws of nature do not become less real! Today's scientific theories about the structure of macro- and microcosm, about subatomic, chemical and biological evolution may well be true. However, the question whether these are primarily material of primarily spiritual processes remains open to debate. Back
N. 10: In any case this could serve as a more plausible explanation than the Christian faith in one almighty, omniscient and at the same time also good God throned in glory over this sadistic madhouse called "earth". Back
N. 11: According to Hegel, man knows of God in the sense that God knows of himself in man. However, God knows very little of himself in man. Within the framework of an Hegelian mode of thought (but transcending Hegel himself), the human species would be considered to be only one transitional stage in (or branch of ) the process of the returning of the spirit of the world (to itself). If mankind exterminate itself before reaching something higher, such as homeostatically intelligent machines or transhumans, it will be no more than a tragic dead end street. Back
N. 12: Cartesians and Occasionalists can solve this problem only with the help of quite ridiculous artificial constructions. Descartes referred to his so-called "Zirbeldrüse" (I don't know the english word for this and I don't know the latin word either) in which spirit and matter allegedly got in contact with each other. Occasionalists claimed that God was permanently busy at all ends of the world producing material motion to fit spiritual processes. Back
N. 13: This view underlies, for example Leibnitz' theory of monads and Schelling`s philosophy of identity. Back
N. 14: I want to summarize my points against a purely scientific view of the world here:
Pascal hinted at the historically relative nature of science when in the debate with the Cartesians he pointed out that the knowledge of our predecessors does not simply become useless although we revise it to a considerable extent. We transcend the knowledge of our predecessors only since we are "standing on their shoulders". However, in the same way in which we transcend our predecessors our knowledge will be transcended by those succeeding us. (Scr.: Höffe, Otfried, ed., Klassiker der Philosophie, Verlag C. H. Beck München 1985, vol.1, pp.330). Back