Body-soul dualism or "interactionist substance dualism" probably has its roots in ancient Greece, but today it is mainly attributed to René Descartes (1596-1650). It refers to the so-called body-soul problem or, from today's perspective, probably more correctly the body-mind problem. Basically, it is about the fact that body and mind seem to be fundamentally of a different kind. The body is clearly part of the "normal", physically describable external world and can be described unambiguously in scientific terms. The mind, on the other hand, seems to obey its own laws and can only be described physically to a limited extent. To date, there is no scientific theory that can explain the phenomenon of consciousness in particular. In the case of body-soul dualism, there is also a metaphysical level of meaning. Only if the spirit as soul can exist independently of the material world and thus of the body are most religious systems even conceivable.
The core thesis of body-soul dualism is as follows in slightly abstracted form:
Assumption 1: There is an indubitable, non-material and non-extended "thinking substance" (res cogitans), which is often also referred to as the soul. This soul, in short, comprises mind and consciousness and does not necessarily obey the laws of nature.
Assumption 2: There is an extended and "material substance" (res extensa) existing independently of this soul, to which the human body or corpse also belongs. This world necessarily obeys the laws of nature.
Assumption 3: These two substances can exist independently of each other.
Assumption 4: These two substances can interact causally with each other.
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Assumption 3 is central to many religious ideas. Only in this way, for example, can the soul undertake a transmigration of souls or continue to exist after death in paradise or hell, but the world also does not cease to exist with the death of one (or the last) human being, as in "idealism.
Assumption 4, in turn, is central because without causal interaction the concept of two substances makes no sense. What good is a soul or a spirit if they cannot act on the body and, for example, move the hand at will? Moreover, it is obvious that matter can have an influence on soul or spirit: the (material) aspirin has an effect on the conscious sensation of the headache.
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Body-soul dualism assumes that all four assumptions are true at the same time. It therefore states that body and soul belong to two fundamentally different substances (hence dualism = "containing two"), but that these can interact causally.
Arguments for dualism
Intuition
The intuition of many people that there is a soul that exists independently of the body speaks in favour of body-soul dualism. This intuition is essentially based on the hope that what constitutes us continues to exist in some form after death.
Of greater importance from a scientific perspective is the argument that the spirit or soul does not seem to be anything material, physical, and can hardly be explained physically. Here, a further differentiation is worthwhile. While the idea of a soul is a metaphysical idea that by definition cannot be fully physically grasped, there are areas of the mind that do not appear to be material, but which can be physically grasped and clearly obey physical laws. Computers, for example, process information that appears to be just as non-material as the mind, which means that there is at least something "spiritual" about it. The big difference to the human mind, however, is that as far as is known today, only living beings have phenomenal consciousness. That is, the subjective perception of how something feels. So we can physically analyse a bat and find out pretty much everything about it - but we will not be able to explain with physical methods how it feels to be a bat (argument by Thomas Nagel).
Body Awareness Problem?
While the concept of the (material) body seems relatively clear, that of the soul is unclear. Does it refer to a metaphysical substance, the spirit that can be at least partially grasped by natural science, or the phenomenal consciousness that cannot yet be explained physically? Assumption 1, that there is an independent "spiritual" substance, must therefore be differentiated here. If the soul includes both spirit and consciousness, then the assumption becomes contradictory if the spirit can be at least partially explained physically. Instead of a body-soul or body-mind problem, one would perhaps better speak today of a body-consciousness problem.
That body and consciousness are essentially different seems so obvious that the idea that consciousness could be something material seems downright grotesque. It is completely obvious that the perception of feelings, thoughts, sensory stimuli etc. is nothing material. So even if one has to differentiate dualism today, the felt intuition of at least one body consciousness dualism seems simply unquestionable.
Conceivability argument
Another argument put forward in favour of a body-soul dualism is the "imaginability argument". According to this argument, substance dualism can be imagined and - so the argument goes - what can be imagined is also possible in principle. Principle possibility, however, does not mean that it really is - and there are important arguments that speak against the imaginability argument (cf. on this "David Chalmer's zombie argument against physicalism). Moreover, it can also be questioned whether something can really be imagined that would have to be part of a completely alien substance. This concerns the problem of the causal interaction of two completely different substances, which is dealt with in more detail below.
Arguments against dualism
No soul without a body
A first argument against dualism is the fact that there is little or no knowledge about a soul that exists independently of the body. These are (almost) exclusively speculations or intuitive insights that do not stand up to empirical testing.
(Almost) all knowledge about the soul comes "from the time" when the soul is bound to the body. Even esoteric "regressions" into supposedly other lives take place at a moment when the corresponding soul is bound to a body that is, for example, in an armchair. The soul, which is supposed to remember previous lives as a result of the therapy, remains in its body. The fact that these are not true memories, at least for the most part, is shown by the simple example that very many women put themselves back into Cleopatra - Cleopatra must therefore have been a truly multiple person (Stamm 2000, p. 84f).
In most cases where a soul is felt to be independent of the body, mind-expanding drugs or rituals are involved. Since these work in the material brain, they seem to be deceptions, at least as a rule. Even the few near-death experiences that cannot (yet) be explained in a natural way leave little doubt that consciousness without a body cannot (yet) be proven and probably cannot exist. For the assertion that souls exist independently of the physical world is a very strong claim that requires more than a few subjective reports of experience (e.g. Eben 2013, Engmann 2011).
All the more so as neuroscientific studies show that the connection between brain and consciousness, between body and soul, must be very intensive (cf. Roth: Relationship between consciousness and brain). Thus the soul sees nothing without eyes, hears nothing without ears, but also sees and hears nothing without the brain. A soul without a body would therefore have no perceptions. Some esotericists therefore assume that the soul has an "astral body" - although this is purely an assertion. The problem with this is that the non-physical "astral body" would have to function in exactly the same way as the "physical" body and would thus make the latter superfluous. It is also difficult to imagine why the soul sees in the body with the eyes of the body and the moment it leaves the body with the "eyes" of the astral body. Quite apart from the fact that such an interaction would have to consume energy and thus be measurable.
However, the soul is not only dependent on the body for perceptions, but memories are also demonstrably stored in the brain, brain damage can lead to changes in personality, hormones and other material substances determine our emotional life. The activity of consciousness can also be shown through "brain images", it is even already possible to generate images based on brain activity alone, which are amazingly similar to the consciously experienced images (lecture by Prof. Dr. John-Dylan Haynes at the Tower of the Senses Symposium in Nuremberg 2013).
The connection between brain and consciousness, or soul, is demonstrably so close that hardly any area remains that the soul could encompass even without a body. Nevertheless, there are some scientists whose books are seriously discussed and who deny the connection between soul, respectively consciousness and brain. In fact, many of these authors argue for a body-soul dualism, although at the same time they distance themselves from such a dualism. This can be explained by the fact that many of these authors argue against a "non-dualism" primarily for ideological reasons, but in doing so get tangled up in contradictions. Examples of this can be found in the following articles: "Bennett/Hacker; "Alva Noë; "Brigitte Falkenburg; "Neuroconstructivism
Two substances?
Descartes had distinguished the two substances primarily by their "extensibility": bodies can be touched and occupy a space, whereas the spirit does not seem to be extended, or at least cannot be touched. In addition, an essential part of the distinction was that the spiritual belongs to a soul, which does not or at most only partially obey the laws of the physical world. As shown above, this distinction can hardly be maintained today. Most of the spiritual never becomes conscious and seems to have great similarities to other information-processing processes that also exist independently of humans.
But if one draws the dividing line between body and consciousness, one can continue to speak of a possible dualism, but this then has little to do with the body-soul dualism. Consciousness may be something fundamentally different from matter and information, but not a substance in its own right. The dividing line between unconscious information-processing processes that can be clearly assigned to the brain and consciousness is too diffuse for this.
Problem of interaction
But no matter where the dividing line is set, whether one wants to hold on to body-soul dualism or understands it as body-consciousness dualism, both sides of dualism must be able to interact with each other. That is why we speak of an "inertialist substance dualism". It is quite conceivable that there are different substances. But then, by definition, we can only know something about the substance to which we belong. Unless there is a connection between the substances, unless the substances can interact. Descartes assumed that this interaction takes place in the pineal gland.
In the pineal gland or wherever, the substantially soulful, non-physical, conscious decision would therefore have to be able to have an effect on the substantially physical world. The conscious decision made in the substance of the soul would have to have an effect on the hand belonging to the substance of the body in order to be able to lift the glass. And conversely, the sensory stimulus belonging to the substance of the body would have to cause the conscious sensory experience existing in the substance of the soul.
Both substances must therefore be connected to each other via a common causality. With this, however, it hardly makes sense to speak of different substances. Body and soul, body and consciousness may be different in essence, but they seem to follow essentially common laws. However, the idea of body-soul dualism is different here.
Supposedly, the soul is supposed to come from "another world" that obeys its own laws. It is said to exist independently of a physical body, to "dock" with a body of the physical world at a certain point in time and to take control of it. It should somehow nestle in the brain (or somewhere else in the material world) and perceive the physical world via physical sense organs - without, however, being physical itself. After all, it belongs to another substance. It would be integrated into the physical causality by perceiving the pain caused by the toe stubbed on the stone, but conversely it would also be able to have a physical causal effect on the world by means of acts of will. With death, however, it would leave the physical body again and continue to exist in its own substance, again completely detached from the material body. An interaction of souls after death is claimed in many religions, but it has never been proven and such an interaction would have to consume energy and thus be measurable.
The idea of a soul connected to the brain, which perceives like a little "human being" (= "homunculus") through the sensory organs of "its" body can be empirically excluded (Singer: homunculus). There is no "place" in the brain where an interaction between material brain and non-material soul or homunculus could take place. Moreover, the causal interaction between material brain and non-material soul would mean a violation of the second law of thermodynamics, the law of energy.
But once we assume that an independently existing soul could causally interact with the world, a problem with logic arises. Is this causal interaction non-physical ("soul") or physical? If a non-physical cause (e.g. a thought) had the physical effect of me moving my hand, it would have to be possible to transform the non-physical into the physical. A non-physical cause would therefore have to be able to have a physical effect and vice versa. If this were possible, the addition "physical" or "non-physical" could be omitted, since it would be a matter of compatible causality. This would in turn mean that the non-physical would be embedded in the physical causality - or the physical in the non-physical causality. The soul / consciousness could thus not exist independently of the physical world, would simply not be an independent substance, but something non-material that would be causally connected to the physical world.
An image can illustrate this thought. If the non-physical causality is a railway line, the physical causality is, for example, a road. If there are hybrid vehicles, then the railway line and the road form a unit and are not completely separated from each other. However, if there are no hybrid vehicles, it is not possible to switch from the road to the railway line and vice versa.
Analogously, body and soul are either really fundamentally different and therefore cannot interact or they can interact and it is not a matter of substance dualism. It does not matter how different body and soul are. If they can interact, it is a system, body and soul belong to the same causal connection and body-soul, or interactionist substance dualism, is wrong. But if they cannot interact, the spirit cannot act on the body, the body cannot act on the spirit, a conception that is empty, so to speak. This would be pure solipsism, the whole physical world would then only be the imagination of my soul and would not exist in reality.
Alternatives to body-soul dualism
If body and soul are not substances in their own right, everything belongs to one substance. This means either that spirit, soul or consciousness are physical in essence or that the physical is spiritual, mental or conscious. The first version is called "naturalism, physicalism or (obsolete) materialism. The second version is "idealism. Theoretically, another position would be possible: "panpsychism, which, however, is contradictory in itself. From a scientific point of view, only one of these versions can be considered at all. For only one of these theories is compatible with scientific theories, especially with the theory of evolution.
Evolution
According to this theory, the world has evolved independently of spirit and consciousness, independently of a soul, and has produced complex organs that have consciousness. According to the theory of evolution, the earth is only a tiny planet in a huge universe, which is why it would be all the more astonishing that souls "cluster" here. The theory of evolution also makes it easy to explain why consciousness always seems to be tied to a brain - if souls could perceive and feel without a body (with an "astral body"), then they could also settle somewhere else. In fact, there are theories that even attribute a soul to stones, but there is no empirical confirmation of this and the question arises as to what else one could even imagine under such souls. This idea is more problematic, however, for ideas that assume transmigration of the soul in whatever form: since the stone does not die, the soul would probably be enclosed in the stone forever.
Further arguments
John R. Searle argues against dualism as follows:
"The fact that the causal forces of consciousness and the causal forces of its neuronal basis are exactly the same shows that we are not talking about two independent phenomena, consciousness and neuronal processes. If two things exist independently of each other in the real, empirical world, then they must have different causal powers. But the causal powers of consciousness are exactly the same as those of the neuronal substrate. The situation is the same when it comes to the causal forces of solid objects and the causal forces of their molecular constituents. We are not talking about two different entities, but about the same system at different levels." (Searle 2006, p. 138f.).
Ansgar Beckermann argues as follows:
"Problems of interactionist dualism
(1) An effect of the mind on the brain cannot be empirically proven.
(2) There is no comprehensible answer to the following theoretical questions:
(a) Why are the effects of the mind so minimal and limited to certain areas of the brain?
(b) How is an effect of the mind on the body compatible with the physical conservation laws?
(c) Why does the mind require a complex and functional brain in the first place in order to be causally effective?
(d) What is the mechanism on which the causal relationship between mind and body is based?
(e) Why can my mind act on my brain but not on any other brain? "
Beckermann 2008, p. 56.
Mario Bunge summarises his arguments against dualism as follows:
"The first weakness of dualism is its vagueness: it does not say what the spirit is because it does not offer a theory of the spirit, or even a definition. At most, it says what the spirit is not, namely not material, not spatial, not localisable, etc. ... Due to its complete ontological lack of clarity, dualism not only does not provide an answer to the question of how the mind is supposed to interact with the brain, but also to the question of why my mind only interacts with my brain and not with several, indeed why it interacts with the brain at all instead of with the liver or the heart or completely different objects (see also Beckermann 2001). A third defect of psychophysical dualism is its incompatibility with evolutionary theory. If the mind is immaterial, then it literally stands above living matter and can therefore hardly be subject to processes of selection and evolution. In contrast, mind as a brain function can evolve along with the brain. The worst characteristic of dualism, and probably decisive for its bankruptcy, is its scientific sterility: neither does it prove heuristically fruitful for research nor can it contribute anything to the explanation of the mental. In fact, it already has an answer to every problem: the "activity" of the immaterial mind. What new research should he thus inspire? At the same time, with his all-purpose answer, he explains everything that ever needs an explanation. But something that explains everything explains nothing. Thus he is inadmissible as an explanatory authority and also superfluous. Last but not least, dualism is untestable, or unfalsifiable:" Bunge 2004. p. 145f.
According to John R. Searle, however, dualism cannot be conquered:
"Many, if not most, philosophers have renounced dualism. ... I suspect dualism will not disappear, even if it has become unfashionable. In fact, in recent years dualism, at least property dualism, has experienced something of a renaissance, partly due to a resurgence of interest in consciousness. The insight that drives dualism is powerful. Very simplistically, it is: We all have real conscious experiences and know that they are not of the same kind as the physical objects around us." Searle 2006, p. 54.