'Heidegger's notion of Dasein is not a solution to the problem of scepticism regarding the external world posed by Descartes but a rejection of the question' — Discuss
In discussing the question, I will first sketch out Heidegger's basic structure of Dasein, outline the problem posed by Descartes and then discuss how Heidegger views the Cartesian model in the light of his own analytic.
Dasein, the Being of human beings, is distinctive in that its Being is an issue for it, Heidegger says. Its life, unlike the life of animals (or other entities such as chairs for that matter) is something with which it must concern itself: "Dasein is in such a way as to be something which understands something like Being... it does so with time as its standpoint" is Heidegger's first tentative, yet affirmative outline of the subject of Being and Time. 
What will most concern us here is the basic structure of Dasein that he outlines: Dasein is Being-in-the-world. Heidegger is at pains to undertake a careful analysis of this ontological structure as it is the foundation stone of his subsequent analysis. He is all the more careful to do this as, in his view, this basic structure is one that has been 'passed over', presupposed, taken as self-evident or misinterpreted by the philosophical tradition — including by Descartes: therefore the nature of Dasein has never been 'revealed' or 'disclosed' or really 'known' before until now. And since "Knowing is a mode of Dasein founded upon Being-in-the-world... this basic state of Being-in-the-world must be interpreted first". Otherwise, it is the 'world' which gets passed over and consequently being -in- the- world, and therefore something essential about the Being of Dasein. Why 'knowing' does this we will see shortly.
Being-in-the-world is a "unitary phenomenon" but Heidegger first attempts, in a phenomenological way, to allow 'world' and 'being-in' to 'show' themselves so that we can see more clearly what these mean for Dasein "pre-eminently in their everydayness".
Heidegger sets out different definitions for what 'world' may mean, but the crucial one for his project is that where "'World' is "that wherein Dasein as such can be said to 'live'. By this he means the particular environment in which we find ourselves just at this time, the world of home and work and social life. It is here that Dasein encounters entities in a concernful way. These encounters are what make Dasein's Being "come alive". These 'ready-to-hand' entities or 'equipment' for writing, sewing measuring and hammering for example are entities that 'are in order to' allow Dasein to do something; they are not mere 'things', their being is 'for the sake of'. Furthermore, equipment exists in a 'totality of relationships' that ultimately forms the web of Dasein's world. Thus a hammer is for hammering nails within a workshop that produces a shelter for Dasein and, by implication, others. Dasein is 'in' this world not as a 'sailor in a ship' — which he can leave — but 'in' in this sense is what Dasein is; its essence is its existence in the world, Dasein is therefore 'spatial'. Later this will be contrasted with how Descartes considers the notion of 'space'.
We are so caught up in our everyday dealings with the world that we don't even notice it, except when something we need is missing or broken — then things are not 'handy'; then the assignment 'towards which' we are oriented is disclosed and, in this way, the "'world' "announces itself". A recent trip to an exhibition of Chinese modern art brought this idea of the ready-to-hand-as-missing home. Everyday items such as sunhats, hairdryers, pillows, shoes and toys, made from china clay were scattered around the floor of the exhibition hall, drawing one's attention to them as unready-to-hand and, thereby, the world in which they exist. The prominence given to a 'hammer' resting on a 'cushion' had a particularly eerie Heideggerian resonance. This emphasises why the concept of ready-to-hand is important for Heidegger; through it, the world is revealed: "in anything ready-to-hand the world is always 'there'" — as Heidegger puts it, this world "does not get created for the first time by knowing it nor does not arise from some way in which the world acts upon a subject". Rather, Dasein encounters the world, it has 'significance' for Dasein — a Being whose Being is about being thoroughly' thrown' in the world.
It is not that 'knowing' is not important for Dasein but, in an ontological sense, it is a 'deficient' mode of Being in that it is a 'holding back' from interaction with objects for a particular reason — to check for instance why the hammer is not working as a hammer. When we stop to consider or think about the hammer, then it is, when we 'know' it in this way, present-at-hand, not ready-to-hand. The danger in the present-at-hand mode is that this approach tends to put us in the mode of " returning with one's booty to the cabinet of consciousness ... 'where I do no more than 'think about them". And how do I exit my closed in order to 'know' the thing? How is it that I 'grasp' what this is? For Heidegger there is an intrinsic relationship between 'human being' and world shown in the way that we encounter things in a ready-to-hand way. We do not grasp things just theoretically or mentally but also physically and practically. We are not along- side things the way a wall is alongside a chair. The chair and wall never 'touch' but a human being encounters the chair, touches the texture of the wall, it does not simply 'know' it as an isolated subject meeting up against an isolated object. In this latter way of knowing, we have to think of knowledge as a property or possession of either a subject or an object and we are on the way to scepticism without any way of accommodating how one isolated subject can check its knowledge of the isolated object. But as Mulhall points out Heidegger wants us to consider "knowing [as] ...an activity carried out in a particular context, for particular reasons... and so one node in the complex web of activities that make up a culture and a society". In this way of knowing, the 'world' is still there and in present-at-hand we are 'tarrying-along-side' but still in the mode of 'in-order-to'. This distinction will be important when we come to consider Heidegger's critique of Cartesianism in due course.
So far we have only '[laid] bare the horizon' by outlining what is meant by Being-in-the-world, but it is the foundation of Heidegger's project. He goes on to elaborate further structures and modes of Dasein's being — how in this very everydayness we can lose ourselves in the world of 'they', the world of other people and forget that through our understanding we can project ourselves onto possibilities in an authentic free-to-choose manner, even as we understand that we are beings-towards the non-possibility of death. We can be in an authentic or inauthentic way. We tend to be inauthentic much of the time, Heidegger suggests, caught up in our fascination with the world of objects and other people. It is our 'moods', particularly anxiety, which have the potential to bring us back to a more authentic state in — bring us back to ourselves as beings with possibilities that we can exercise. A further structure of Dasein is temporality; this unites all the elements he has already outlined. In temporality, Dasein is being open to the past (historical), making the present and projecting the future. Dasein can be authentic by grasping the possibilities that are open to it (which are not endless) in the situation it finds itself thrown (into a particular family in a particular culture for instance). But without the notion of the 'world' all of these other structures would not make sense. It is just this notion of the 'world' that gets overlooked, according to Heidegger in the Cartesian mind-body question.
"I shall consider myself as not having hands or eyes, or flesh or blood or senses, but as falsely believing that I have all these things," says Descartes is his First Meditation as he embarks on his project of 'knowing' by "demolishing everything completely and start[ing] again right from the foundations". Thus, from the beginning of his meditations, the very existence of the body gets called into question, and there is a clear separation of mind and body. But even an evil deceiver, the false information provided through the senses and the state of sleeping and dreaming cannot make him doubt that he is thinking: I think, I exist. His foundation for what he can know is therefore what he can grasp with his mind. Descartes now has to as, Cottingham puts it, "find a convincing route outward from the inner prison of subjective cognition to the reliable knowledge of objective reality" (one he has failed to do according to Cottingham). There is no necessary or essential relation between 'the world' and a 'human being' that is a 'thinking thing' in this scenario. Descartes sets out however to try and find one.
Descartes' analysis of the 'thinghood' of a honeycomb of wax in the Second Meditation culminates in the understanding of it as a res extensa — a substance extended in space which is grasped "by the mind alone". When everything else it taken away — smell, feel, colour, the sound it makes when struck — a clear and distinct idea of its extension remains. He can come to this understanding because he is, in essence, a 'thinking thing' or res cogitans: "Even bodies are not strictly perceivable by the senses or the faculty of the imagination but by the intellect alone and that this perception derives not from being touched or seen but from their being understood" This mind is wholly distinct from his body. Even though he cannot be "separated from [his] body as [he] could from other bodies", it is a substance in which the incorporeal mind subsists. The mind then is of his essence while the body is a mechanism which somehow is connected to the mind. The body can of course give him information, but this may be false; if my limb is cut off, I may still have the sensation of pain in it — this, according to Descartes, is more evidence of the unreliability of sensory information and a further reason to make a clear distinction between mind and body. In Descartes' 'world' we can therefore, without doubt, have a res cogitans (a thinking thing) and a res extensa (a thing extended in space).
A substance, which is a res extensa, — here Heidegger quotes Descartes in the latter's Principia Philosophiae — is "an entity which is in such a way that it needs no other entity in order to be". And there are two kinds of substance in the Cartesian world (as we have seen), a res cogitans and a res extensa. What we now have, Heidegger suggests, are two 'present-at-hand' entities along side each other as isolated objects. As we have seen, when all we have is isolated objects, then the 'world' remains hidden and so the being of Dasein as Being-in-the-world gets passed over. Even Descartes himself needs God, 'who is no deceiver' to put clear and distinct ideas about entities into his mind in other for him not to doubt that they exist (and thereby enters the so-called Cartesian circle). But, now that he has found the foundation of knowledge in the incorporeal mind, Descartes can also explain the possibility of the existence of objects through mathematics.
He begins his Sixth Meditation with, "It remains for me to examine whether material things exist. And at least I know they are capable of existing, in so far as they are the subject matter of pure mathematics, since I perceive them clearly and distinctly". Heidegger interprets this to mean that in a Cartesian world "that which enduringly remains, really is. This is the sort of thing which mathematics knows. That which is accessible in an entity through mathematics makes up its Being". The Cartesian world consists then of these entities which have their kind of being prescribed for them, says Heidegger. What he is attempting to do rather is to allow the entities in the world to disclose themselves; they are disclosed when they are allowed to be ready-to-hand as we have outlined. Two present-at-hand entities (such as thinking thing and extended things) cannot be related to each other. He gives the example of Descartes' explanation of the attribute of 'hardness' as 'resistance'. Resistance amounts to no more than not yielding place, not changing location. In this way, Heidegger suggests, the experience of sensory perception is obliterated and things cannot be then grasped in their Being; what we have instead is two res extensa side by side, related to each other by means of mathematical calculation. But, according to Heidegger, "hardness and resistance do not show themselves at all unless an entity has the kind of being which Dasein -or at least something living — possesses" 'Hardness' is something that Dasein encounters in how it experiences the object, hardness is disclosed to Dasein in this way.
For Descartes, space is a matter of abstract mathematical coordinates and calculations in which things are located and move about; for Heidegger, space is how Dasein experiences things. Things are 'near' or 'close-by' according to what Dasein is concerned with at any one time. He cites the example of how the spectacles on our nose can be considered further away than the object in the 'distance' that we want to give consideration to in some way. As Mulhall points out, what Heidegger is saying here is that " space and spatiality are thus neither in the subject nor in the world, but rather disclosed by Dasein in its disclosure of the world". It is this 'disclosure' of the world that readiness to hand provides. In disclosure the world is revealed or unveiled (but there already) by Dasein. In the Cartesian analysis the world would have to be 'added on' to the life of humans, in the way that the body is somehow tacked on to the mind of the human being. In this analysis objects are primarily encountered in a present-at-hand or in an isolated, decontextualised way and the 'totality of relationships' that make up the world of Dasein is not actually encountered and therefore the Being of Dasein is not encountered. The world for Heidegger is not a series of objects that we can come to know but as Mulhall describes "a web of socially or culturally constituted assignments within which entities can appear as the particular types of object that they are and which must be disclosed in advance of any particular encounter with an object". He cites the example of learning to drive a car — it is in this experience that entities such as a wheel or a kerb or gear stick can appear as the entities they are. Eventually, all these entities become inconspicuous as we become absorbed in them and we don't notice our 'world' or the 'worldhood of the world'. This is why it tends to get passed over and why priority tends to be afforded to 'knowing' as it seems almost more obvious.
It's important to emphasise again that, for Heidegger, it is not that the present-at-hand or decontextualised mode of encountering the world is not a valid one; rather it is a deficient mode and, as such, not the primary mode we should use in encountering the world. Since this is the starting point for the Cartesian mode of analysis, the analysis must be deficient. It is deficient in that it if this is its only — or at least primary — mode, it cannot account for the ready-to-hand as a grasping or constituting of in-order-to and thereby misses both the world as 'world for Dasein' and Dasein as a being whose being is an issue for it, i.e. the possibilities it has through practical engagement with the world.
This is why we can say that Heidegger's notion of Dasein is not a solution to the problem of scepticism regarding the external world posed by Descartes; he starts from a different perspective. For Heidegger, there is not mind, body, and world, but Dasein in-the-world, as a 'unitary phenomenon'; in this sense, Heidegger rejects the Cartesian question posed.
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1. Heidegger, Martin, Being and Time Blackwell 1962 par 17
2. Ibid, par 62 p 89-90
3. Ibid par 65 p 93
4. Ibid par 62 p 90
5. Heidegger, Martin, Being and Time Blackwell 1962 par 62 p 89
6. Mulhall, S. Routledge Philosophy Guidebook to Heidegger and Being and Time, Routledge 1996, p44
7. Descartes, Rene Meditations on First Philosophy. Trans J Cottingham Cambridge University Press 1986, First Meditation p 12-13
8. Cambridge Companion to Descartes, Ed J Cottingham, Cambridge University Press 1992 p253
9. Descartes, Rene Meditations on First Philosophy. Trans J Cottingham Cambridge University Press 1986, Second Meditation p 22
10. Heidegger, Martin, Being and Time Blackwell 1962 par 92 p 125
11. Descartes, Rene Meditations on First Philosophy. Trans J Cottingham Cambridge University Press 1986 p 50
12. Heidegger, Martin, Being and Time Blackwell 1962 par 95 p128
13. Ibid par 97 p130
14. Mulhall, S. Routledge Philosophy Guidebook to Heidegger and Being and Time, Routledge 1996, p 53
15. Ibid p. 50