What do two people need to mutually know to be able to have a conversation about the same aspect of something?
This is a question that needs a lot of background fleshed out before we can even start to try to answer it. It is an open question as far as I know, but we are getting more information about it all the time from various studies of different aspects of social phenomena. To put it into perspective this is the famous problem of intersubjectivity that haunted Husserl’s phenomenology in the period of Cartesian Meditations. How does intersubjectivity work, is a fundamental question that has been worked on by phenomenology ever since it was realized that it was a problem with phenomenology, it is not just that there are noumena out there that we want to bracket so we can get back to the phenomena itself, but when we bracket those noumena, we are also bracketing the other subjects as well, and that produces an unbridgeable gap between the subjects and makes phenomenology solipsistic. Basically this lead to getting rid of bracketing and the realization that we can posit a world horizon upon which the phenomena appear and that solves the problem. We see this in Heidegger’s Being and Time but evidently he got it from the late work of Husserl.
Once it was discovered that it was possible to achieve the same effect that bracketing does by positing the world horizon as the background for all phenomena, then the road opened for the solution of the problem of intersubjectivity at a theoretical level, because as is obvious our sociality is something that comes out of our interactions in society, which is part of us from the very beginning not something that appears later, as Heidegger says immersion in Mitsein is prior to Dasein. But exactly how to create a social phenomenology has been the subject of intense research since the time of Husserl and Heidegger, for instance Merleau-Ponty made great strides in this direction through his questioning of some of the basic tenets of the transcendentalism that still haunted both the thought of Husserl and Heidegger. Reflexive Sociologists like Alan Blum, John O’Malley, and Barry Sandywell developed a Social Phenomenology which was philosophical within sociology based on the work of Alfred Schutz. One amazing development is the book that Hurbert Dryfus promoted by Samuel Todes called Body and the World (http://www.scaruffi.com/mind/todes.html). Following in the wake of that is the work of Shaun Gallagher, 2005. How the Body Shapes the Mind.
As phenomenology gets into neuroscience we are slowly getting a picture of how we place ourselves in the place of others and understand them and ourselves using concept like mind blindness, and folk psychology, and other ideas that have come out of research into how children come to various stages of their social development as they mature and develop. So there is a lot of background to cover here, and even I do not know this whole literature as much of it is new and it continually needs to be explored to try to keep up.
But what I will do is give you my take on this question which is one worth delving into. In the tradition of Reflexive Sociology, which has been all but forgotten since it as an English and Canadian Movement mostly which followed Merleau-Ponty for the most part in turning the question upside down. In other words the real question is not how we can have intersubjective experience, but how we can get into the position we are in within our own society where we cannot imagine how it is possible due to our long history of individualism and the fear of the masses especially after world war two. In other words, since we are social from our inception, the question becomes how we come to feel isolated as individuals so we can think of ourselves as independent from the social relations we are embedded within even as we think individualistically. Just one fact will show this point, which is that when the baby is not yet a year old and it is in a room where people are talking and we look at its micro movements, we see that its body is moving to the cadences of the different parts of speech with micro movements. Body of the baby tracks the speech that is in its environment even before it can speak, in such a way that different parts of its body move to different levels of speech and what part of the body is constantly shifting (Condon, W. Speech make babies move. In R. Lewin (Ed.), Child Alive. New York: Doubleday, 1975). So if babies are tracking with the movement of their entire bodies speech that happens to be in their vicinity prior to their own speaking we can see just how immersed we are in the intersubjective experience from the very beginning. The real question is how we have culturally separated ourselves back out in the aberration of individualism take to an extreme that we see in Western culture. Intersubjectivity as the Late Husserl, Heidegger, and Merleau-Ponty realized is a given, and what needs to be explained is the ego, (I) as we have socially constructed it in Western culture as the Subject as opposed to Objects.
So you can see that even in the question that is asked here, there is a subtle assumption that there are two people and we have to bridge the gap with knowledge so that they can talk about the same aspect of the same thing.
Rather the situation is that there is in fact something social that differentiates itself out into individuals who are always already the same but who in fact together construct a world by mutual projection so that they are already in synchrony from the beginning. And the question is really how we see them as individuals relating to each other externally though communications channels using reference to coordinate their actions. In reality they are pre-coordinated and that pre-coordination differentiates itself out in our society into individuals but in different societies the basic substrate may be something other than the individual. For instance, in Japan the basic unit of society was the Ie which was husband, wife, first son and his wife. And how you know that this was the individual for that society is that it was this unity that experienced capitol punishment. In China it was a whole clan that was treated as an individual. In fact probably most traditional societies had something other than the single human organism as the basic unit of society. When you are an organ within the social organism it is very different from the corporal body being the same as the social organism as it is in Western society today.
It is funny that the question speaks of knowledge as the glue (as what we have to know) because we now know that this ability to know what others know, comes on at a certain age in childhood. Prior to that the child does not project what the other knows, but once that threshold is crossed then the normal child takes into account what it knows the other knows in the way that it interacts with the other. (http://web.mac.com/jopfer/courses/846-Concepts_files/Flavell (1999) TOM.pdf) So we know this is an important milestone when children can take into account the knowledge of the other in order to coordinate their actions with that of the other. And when this does not happen this is called mindblindness (http://en.wikipedia.org/wiki/Mind-blindness). It means we have no theory of the other being like ourselves, i.e. as thinking and emotional beings whose thoughts we can predict by a folk psychology that is a social psychology attuned to the cultural context. It is interesting that in terms of our taking the point of view of the other that this is helped by mirror neurons when we are watching other people work the same neurons fire as if we were doing the work ourselves (http://en.wikipedia.org/wiki/Mirror_neuron). This explains physical synchronization of action and our ability to coordinate with the other and learn physical tasks by watching. It is interesting that when the other person is pretending to do something rather than actually doing it that this phenomena stops and registers the difference between pretense and what is for real in the actions of others. And finally we know that we are able to carry this taking the place of the other up to about five levels before we start to get confused, about the same number of levels as there are Kinds of Being or Knowledge that we might have (http://www.staff.ncl.ac.uk/daniel.nettle/liddlenettle.pdf). When we take these various advances together we find that they underline the developmental, neurological basis of social interaction, and how it is a primary process, and individuation in the sense of Jung or Heidegger (authenticity) is always a secondary process. Children learn early to take the knowledge of the other into account, and soon after that they are climbing the various meta-levels of the theory of mind (he said, she said, he thinks, she thinks, he believes, she believes, he does,she does) and then all the meta-levels after that i.e. he thinks that she believes that when he acts then she acts because she believes that he thinks, that she acts . . . etc.) The meta-levels are believes, belief about belief, belief about belief about belief . . . etc. But when you mix thinking, believing, acting, saying all together the whole set of relations among the various meta-levels of these modalities becomes a very confusing maze which we are all trapped in as we experience the reflexivity of our existence. Add to that illusion, and delusion when we act on illusion and we get a radically unstable situation for all social relations, and an unexpected depth to which we can reason concerning these intertwining relations. What people need to know in order to communicate about their mutual relations is VERY complex, but we do it effortlessly for the most part because a lot of it is unconscious for us. But of course that only makes it worse because we take it for granted that relating to others in relation to the world comes naturally to us and seems perfectly transparent in most cases because we glide along using our schemas of situations and others until something unexpected happens and we need to pour effort into processing the anomaly in social relations or in our relation to the world. A spouse unexpectedly commits suicide or loses their job, or there is a divorce. Then this complication of the world, and our relations to others comes to the fore and that is when life becomes infinitely more difficult.
Now if we have this kind of Background that says that our relations with others and the world is incredibly complex, then we see almost all philosophies are a vast simplification of this situation. However, Phenomenology as it develops is trying to keep up, and to produce a valid picture of this situation in which we are relating to other subjects and to noumena together. Sartre talks about being-for-others and being-in-itself, i.e. the reification of acting for others to maintain conventions or stability in society, or reifying oneself into an object. And so when we talk about both of these together we get badfaith and alienation that together perhaps produce anomie. For instance, Kristiva talks about the abject. When we think of all the things that can go wrong so we cannot talk about the same thing in the world with another, then we get some idea of this complexity which is covered over by conventionalism and schematization, and nominalism within society.
Then there is also the question of true names, i.e. are we really talking about the same noumena, and are our indications of it true to the phenomena itself. My preferred approach is social constructionism of Berger and Luckmann with all the caveats I discussed in another answer.
But lets go from these background considerations to the point at hand. For me following Heidegger there is no “two people [who] need to mutually know [something] to be able to have a conversation about the same aspect of something?”
The subtext of this question is subjectivity/intersubjectivity as an assumption it seems to me. What we want to do is go beyond that as Heidegger does by appealing to Mitsein and the differentiation of Dasein out of Mitsein. Heidegger is in a sense turning Hegel upside down and starting from Spirit to derive the self-conscious subject, and in that approach the Master/Slave dialectic which Hegel uses as a starting points evaporates away. It is replaced with the Mitsein as Master and Dasein as Slave, and Dasein’s struggle to become authentic by confronting its own finitude. There is no idea in Heiddegger of TwainSein, i.e. two who differentiate out and are authentic together. But this is where I like to appeal to Sadler’s Existence and Love.
If we forget for a moment that Sadler uses the example of Romantic Love which is nihilistic (see Coming to our Senses by Berman for an explanation of why this is, also Love and Limerence Dorothy Tennov) and lets concentrate on his major salient point which is that the main sensory modality taken as a paradigm in existentialism is visual and this leads to its individualistic nature in all the major existentialists, while if we take hearing as the main sensory modality to be taken as the paradigm of our relations with other things and people we get a quite different existentialism. In hearing things are mixed, interpenetrated, sounding together, in sync, in rhythm, having melodies, they are on key, etc. Sadler makes the point that by concentrating on hearing, or listening as the major modality which is to be taken as the paradigm for our interaction intersubjectively and with things in the world then suddenly the distance and separation vanishes because sounds are immediate to us.
So if we take this point seriously and then go back to the question, we can pick out the phrase “what do two people mutually need to know . . .” and we apply Sadler concept of an existentialism that starts with our deepest relationship to each other, rather than our separation and distance. The answer then is that people are intertwined with each other though hearing and they need to know that they are separate. To point out an aspect of the world is to render it present at hand. Merleau-Ponty translates Heidegger’s present-at-hand as pointing, and ready-to-hand as grasping. Everything in Heidegger is in relation to the hand. And that is because for him it is different for a person to touch something in their world than for two things to come into contact, i.e. objective relations are never the same as the pre-subjective relations of Dasein with the world as a whole. Dasein is prior to the Subject/Object duality. Dasein has three existentiels which are talk, understanding and discoveredness. But Heidegger does not delve into the fact that in talk we are listening. We are listening so deeply that our whole body moves with the speech in micro-movements. We are taking into account what the other knows from an early age. We are projecting our theory of mind on the other, when that gets out of hand and we project our theory of mind on everything then we get Aristotle’s reduction of our relation to everything to our social relations. His Categories have to do with possible Speech topics. His Physics assumes that all things act as higher organisms do, i.e. have a telos a goal. We are so intertwined with each other that we go up the meta-levels of the theory of mind as far as we can which can be to about four or five levels of indirection all the time taking into account not just various modalities of interaction but also gender modulations. But in order for us to point to something present-at-hand within the world we mutually project we must separate ourselves from each other and go into a visual modality and a modality related to our hands which point, grasp, bear etc. We point out the same aspect of the same thing to each other, but in order for us to grasp it, to understand it (realize it with the existentiel of verstehen), we have to separate ourselves from it and each other in our differentiation of the world. And that pointing is rending it present-at-hand to us and thus caught up in the subject/object duality. But prior to that there is the technological infrastructure of the world of our circumspective concern for the whole world and how everything works together, hangs together and supports our presentation of one thing over all other things in the world. This is realized though the ready-to-hand of grasping affordances given to us by our world in Process Being. But prior to that is Hyper Being where there is mutual bearing as Levinas describes it between mother and child. The mother bears the child, but the child bears the ministrations of the mother. This mutual bearing is at a point where ethics and metaphysics collapse together. And it is very difficult to separate things because the distinctions keep slip-sliding away. It is the modality I call the in-hand where the tools are not just grasped but transform in our hands. We use tools for other purposes than they were designed for as the situation presents itself in our world (we can call this the McGiver syndrome). At this level it is not just things that are hard to distinguish due to what Derrida calls DifferAnce (differing and deferring) but also our relations with others. Lacan talks about the mirror stage where the infant recognizes itself in the mirror. Prior to that there is no distance between the self and the self, or the self and the other. But once the mirror stage is reached then we recognize the distance between ourselves and ourselves and thus between ourselves and the other. Distance comes into play and we realize that essential separation that we will need to pick out aspects of things in the world and point to them in the present-at-hand modality. Zizek says Lacan and Derrida are duals of each other, and while Derrida opens up what Plato calls the Third Kind of Being (Timaeus) Lacan explores the mimesis that goes on in that space of distance and separation and its implications. The most significant concept that comes out of that is the idea of the Floating Signifier, which is the Name of the Father, which signifies this separation because the father produces distance from the mother. We can see this in the fact that fathers take the child from the mother and play games like throwing it in the air and catching it in order to introduce thrill in the child. That thrill is positive for some children and negative for others in whom it induces fear, and even trauma. Deleuze takes this concept of the floating signifier seriously in his book the Logic of Sense.
But as Merleau-Ponty pointed out in The Visible and the Invisible Hyper Being (the hyper dialectic between Heidegger’s Process Being and its antimony Sartre’s Nothingness) has a dual which he calls Wild Being. In wild being we are encompassed as Heidegger points out in the Mitsein, we can read Cannetti’s Crowds and Power to understand this fear of the masses and mass movements. One of the best ways to see this is a book called The Attack of the Blob: Hannah Arendt’s Concept of the Social but we can also see it in Jungs concept of individuation. In the first part of the century there was a real fear of mass movements due to the rise of Fascism and Communism as Ideologies. However, we must admit that there is a kind of Being, called Wild Being in which we are always already encompassed by the other, as before the mirror stage, and this primal encompassment which goes all the way back into the womb is the underlying situation that we return to when things get out-of-hand within our world and we are overwhelmed. Prior to that is the singularity of all lost origins which we can call Ultra Being which shows up as our unique fated existence based on our DNA, our situation in time, with these parents, i.e. all the things that make us unique and singular which we have absolutely no control over but which the existentialists focus on as being prior to everything else, and according to Sartre is the basis of our freedom.
So what we have to know is that our world is constituted intersubjectively and that our world has meta-levels of Being which we inhabit without realizing it, and there are levels beyond the present-at-hand in which we are trapped most of the time which is rulled by onto-theological metaphysics (Heidegger) and logocentrism (Derrida). But the pointing at something in the world is dependent on distance, and distance is exactly what we have to constitute in order to be separate from the other and from the things in the world. So we have to know that there is this hidden depth were separation and distance disappear, and we have to continually reconstitute that distance in order to realize the present-at-hand in which we can point to aspects of things hand have those recognized. We wont get into the fact that this pointing is setting up a sign and therefore evokes semiotics as a threshold to linguistics and Symbolic Interaction (G.H. Mead). We have to know that there are layers of the world where we lose the ability to distinguish and that we loose all distance, and then we have to know that we reconstitute separation and distance, and difference as an ongoing process in the way that Deleuze discusses in Difference and Repetition. A good primer on how the world looks at the level of Wild Being is Anti-Oedipus and A Thousand Plateaus by Deleuze and Guattari.
To summarize we need to know the structure of the world that we are embedded in and how we produce it unconsciously together out of gatheredness as it flows into separation and back into gatheredness. And these two people (who may be strangers or lovers, OR strangers and lovers) know this not explicitly but implicitly through a tacit knowledge of an implicate order within the world that allows all these emergent meta-levels of Being unfold and be lived in without our being aware of it consciously.