Metaphysics Revisited
Ontology is the study of being, or what there is. Insofar as it is concerned with what there is, ontology amounts to counting entities. You might count entities at different levels of reality, e.g. tables, people, atoms, and consequently, have ontologies at different granularities. Metaphysicians are often concerned to study the fundamental or base level of reality, i.e. being or beings that underwrite electrons, quarks, etc. Ontologies at higher levels of reality are derived, or applied ontologies. In Being and Time Heidegger restricts his attention to the fundamental level of reality.
This is not, however, to say he is studying fundamental ontology. For there is more to reality than simply counting beings in it, namely, there is also how such beings are structured and relate to each other. Call the study of structure ideology, and consider an analogy to keep these ideas straight: ontology is to ingredients in a recipe, as ideology is to how to combine ingredients in a recipe. You'd count cloves of garlic in a dish, but not mincing. Similarly, you'd count tables and colors in an ontology, but you'd not count as an entity the way a color inheres in a table. This is all to say I take it Heidegger is interested in ideology - the structure of being - rather than ontology - beings. He says as much, though he uses confusing language (he claims he's interested in "Ontological" rather than "ontic" reality; "ontic roughly maps to "ontology" in my sense, and "ideology" in my sense to Heidegger's "Ontology").
Epistemological Puzzles?
In particular, the structure Heidegger is attempting to uncover is the structure of the most important being of all - Dasein - us! That seems a reasonable place to start, given we're of course always viewing things from a subjective perspective, and fundamental metaphysics is no different. But before we can even begin, a host of perennial philosophical puzzles arise. Consider the so-called Problem of Other Minds:
I've direct access to my mind
If (1), then I can be certain I have a mind
I've only indirect access to the 'minds' of others
If (4), then I cannot be certain others have minds
If (1) - (4), then I cannot distinguish between worlds in which only I have a mind and worlds in which I have a mind and others have minds
Hence, I cannot distinguish between worlds in which only I have a mind and worlds in which I have a mind and others have minds
This is quite a puzzle; if (6) is true, then you've no justification - it seems - for claiming to know other people exist. That sounds like philosophy gone crazy. A common response was offered by Bertrand Russell in The Problems of Philosophy. It is to question (4) by asserting that even though we only have indirect access to the minds of others, we can be certain there are other minds by analogy. It doesn't hurt me if I hit you with a hammer in the foot, but you behave as I would. You also look like me, etc. This response by analogy seems plausible, but also weak. Analogies are notoriously susceptible to perspective influence and error. Another response - offered by Heidegger's teacher Husserl - denied (3), claiming that as a matter of fact we do have direct access to other minds. That seems…false…Heidegger thinks the argument itself is ill-posed since he think we're - Dasein's - obviously already encountering the world with others in it. Indeed, it seems we learn about ourselves long after being thrown into it populated by others. This suggests one might start from the opposite direction, deriving the individual from the collective group to which they belong.
More formally, the shift in perspective might be elaborated by supposing you've the task of describing a domain of squares. Squares have points and four lines meeting at right angles. You can approach this task in at least two ways. You might take a mereological starting point, i.e. working from the parts of squares to the whole square; you might take a topological starting point, i.e. working from squares to lines and points. More concretely, suppose you approach this mereologically, say, with only points. Then you can assert that between any two points is a line, some lines form 90 degree angles, and when four such lines form 90 degree angles a square is formed. Suppose instead you approach this mereologically. Then you begin with squares, and you can assert that where any two squares meet there is a line, and where four squares meet there is a point.
One might apply this to the case of Dasein, that entity always discovering itself as a member of a larger group long after becoming a member. This avoids The Problem of Other Minds above, since we seem to - as a matter of fact - have access to other minds once we find ourselves as individuals. Unfortunately, this can't be the story, since it simply reverses the above concern. Call this the Problem of My Mind:
I've only indirect access to my mind
If (1), then I cannot be certain I have a mind
I've direct access to the collective mind
If (4), then I can be certain there is a collective mind
If (2) and (4), then I cannot distinguish between worlds in which there is only a collective mind and worlds in which I have a mind and there is a collective mind
Hence, I cannot distinguish between worlds in which there is only a collective mind and worlds in which I have a mind and there is a collective mind
This is simply the inverse of the Problem of Other Minds. The problem is that starting from a collective and inferring the existence of an individual runs into the same epistemic worry as starting from the individual and inferring the existence of other minds. Now, you might think this argument is not a strong as The Problem of Other Minds. You might think you've more evidence that you have a mind than that others do. Consider though, many of us trust physics to explain the natural world. Many, moreover, have observed free will - voluntary action of agents - is outside the scope of physics, and is certainly not postulated by any existing physical theory. And from this many conclude there is no such thing as free will. Rather, every decision you've made is explained by particles bouncing off each other, as well as beliefs and desires you never chose but realize you have. This, indeed, is a common position. Note the parallel. If you're willing to infer from the apparent non-existence of free will in the natural world to the lack of free will in your mind, then why not similarly infer from the apparent existence of collective minds to the existence of your own. This is precisely the sort of inference free will skeptics make.
***Response: You've more evidence that there is free will than that there's not, since you seem to engage in free actions all the time. If you're saying there is no free will because physics doesn't say there is, then you're inferring from how things seem from the outside to how things seem on the inside. That is, you're letting the outside world tell you you're wrong about your experiences.***
Is Heidegger susceptible to this problem? Not at all. True, one might start from mereology or topology to model the fundamental ideology of Dasein, but doing either leaves you with an epistemological puzzle. Heidegger thinks the problem is starting from the epistemological stance in the first place while trying to do metaphysics. Naturally, he suggests starting from the metaphysical perspective. And with that, Heidegger claims Dasein and the collective group to which any given Dasein belongs, are mutually dependent on each other, the same way hue, saturation, and brightness are mutually dependent on each other; you can't have one without the other. This is to say, Heidegger rejects (2) and (4), since it presumes a distinction without a difference.
Dasein
Epistemological puzzles aside - for now - we have the assumption of ourselves - Dasein - as exhibiting a three part ontological structure, much like color exhibits hue, saturation, and brightness, and this structure intimately involves collective groups we find ourselves members of. Specifically, Heidegger claims Dasein has been thrown into the world, i.e. influenced by culture imposing dispositions on us as we develop, is presently fallen, i.e. finds itself as a member of a larger group imposing such things on it, and projects to the future, i.e. examines options for how we want our lives to look going forward. The temporal references here are important (this is being and time after all); Heidegger claims time is the form Dasein takes.
We eventually realize we've been thrown into this world, raised by people, adopted beliefs, desires, etc. and experience something like a rising self-awareness. It's like one day you're looking around and realizing you're looking around! Then you realize you've been looking around. But then you realize you have to keep looking around since that's one of the things you do. Importantly, you continuing to look around implicates you in your past and present behaviors. Heidegger calls this being guilty, but it seems to me something more like being responsible. Suppose you've been tapping your foot for a bit in class but haven't realized it until someone points it out to you. Suppose you're a bit embarrassed and don't want to admit you've not been paying attention to your behavior, so you keep tapping your foot. Your tapping after you realize you've been tapping implicates you in your previous tapping. It's as if you become retroactively responsible for the previous tapping, by intentionally continuing to tap. Projecting to the future is like that. We all project and plan for the future, but in doing so we all recognize we've been influenced by culture, etc. By pursuing things like money, fame, respect, etc., we implicate ourselves in past behaviors that align with those pursuits. That’s being guilty; taking responsibility and ownership for your past and present, while keeping an eye to the future.
Epistemological Puzzles Revisited
So far, so good. But Heidegger isn't out of the metaphysical woods yet. He's engaged in fundamental metaphysics, but given each Dasein operates within a limited subjective perspective, this sort of endeavor is susceptible to the question "How do you know that fundamental metaphysical view is true?" More concretely, Heidegger is open to the question "How do you know your characterization of Dasein and its complex ontological structure is true?"
Many philosophers attempt to bridge the gap between epistemology and metaphysics by appealing to something like the following principle:
(IMG) If S imagines P then P is possible
Note first this links a faculty of the mind - the ability to imagine - to whether certain things are possible or not. Possibility is under the purview of metaphysics, and exploring what is possible is a practice many engage in when trying to determine what is actually true. Recall, metaphysicians can't go test claims empirically, they need to use reason and human faculties. If they could test things empirically, they'd know what was possible, since whatever is actually the case is possible. Unfortunately, the other direction doesn't hold. Still, it's hoped that if you can determine what's possible, that's an important step in narrowing down what, say, the actual metaphysical structure of the world is like. Important for us, Heidegger first investigates what is possible before turning to what is actual. Doing that involves something like (IMG).
But is (IMG) true? That depends on what's meant by "possible." Note, we're doing fundamental metaphysics here, so we mean metaphysically possible, i.e. any possible way the world could've been. For example, I could've had hair. That's metaphysically possible. On the other hand, it's not true 2+2=5, and could never be. That's metaphysically impossible. Metaphysical possibility is contrasted with other sorts, e.g. epistemically possible. This is possible with respect to what you know. For instance, Lois Lane believed it epistemically possible that Clark Kent was weak. But that's not metaphysically possible, since he's Superman, and Superman is quite strong.
Returning to (IMG) and thinking about metaphysical possibility, it should be clear (IMG) is too strong. Simply put, I could be mistaken about my imagining. One way to fix this is to introduce a qualification to imagining. For example (taken from Descartes):
(IMG*) If S clearly and distinctly imagines P then P is metaphysically possible
This runs into all sorts of counterexamples not worth discussing. Important for us is the natural move philosophers make to fix principles like this, namely, to fill in the blank:
(*) If S ____ imagines P then P is metaphysically possible
One might refine the type of imagining as Descartes did, or perhaps even try out a different attitude, e.g. supposes, but Heidegger takes a different tact. He appeals to mood. Moods like, say, anxiety, aren't like attitudes. To see why, simply observe one can imagine anxiously or not, or perceive anxiously or not. In this sense, moods are ways in which one engages with the world via attitudes. It's, moreover, independent of content. One might imagine an empty room anxiously or not, and get a different experience each time. And the difference in experiences given the same attitude and content is crucial for why Heidegger thinks moods help here. For he claims anxiety is a mood that discloses the fundamental structure of Dasein. In other words, we have:
(ANX) If S anxiously imagines P then P is metaphysically possible
Now, there's surely an epistemological puzzle to be explored here. Before turning to that, however, we should get clear on what this anxiety thing is. As stated, it's a mood, so that's settled. It's not supposed to be - in Heidegger's understanding - feeling upset or negative, and it's not even supposed to have an object, i.e. something to be anxious about. It's more like a calm state of realizing you're groundless, you're on your own, and you've to accept everything you've done and are doing or not. It's the moment of openness to the possibility of the your future as an intentional agent, while accepting responsibility for every disposition imposed on you as you move forward.
In that respect, anxiety is supposed to be clarifying, the truest mood, the one that discloses reality. It's seeing clearly in a dark room; having a Eureka moment solving a puzzle; etc. Anxiety is that moment you - Dasein - really gets it.
So, do you think (ANX) is true? Consider the sort of argument Heidegger needs to get his metaphysical picture out of the concern that, well, he could be wrong given his limited, finite, epistemic state (let "P" be Heidegger's entire fundamental metaphysical view; let "K" be an operator on sentences that means "S knows that", e.g. "K(P)" means "S knows that P"):
If S anxiously imagines P, then K(S anxiously imagines P)
If S anxiously imagines P
K(S anxiously imagines P)
K(If S anxiously imagines P then P is metaphysically possible)
If K(S anxiously imagines P) then K(P is metaphysically possible)
K(P is metaphysically possible)
P is metaphysically possible
Not all steps are needed here, but I include them for clarity. Consider (1) is needed to link imagining anxiously to being in the right mental state, i.e. knowing, to even recognize that you're anxiously imagining. This is plausible since it seems if anxiety is going to be useful to exploring fundamental metaphysics, then you should recognize anxiety when you're in that mood. (2) is granted for the sake of argument, and (3) follows from (1) and (2) by logic. (4) is the principle (ANX), but again it is only useful if the relevant agent recognizes it's true, since otherwise (ANX) is useless to exploring fundamental metaphysics. (5) follows from (4) by logic, and so (6) follows from (3) and (5) by logic. (7) follows from (6) because 'knows' is factive which is to say if you actually know something then it must be true, i.e. you can't know false things.
Do you think this argument is sound? If so, then Heidegger has no epistemological concern in this area, and anxiety gets us to the nature of the world. If not, then Heidegger may be in trouble, but you'll need to identify where the argument fails. Don't try to dispute the logic, i.e. (3), (5), and (6). I'd be inclined to grant the move from (6) to (7), and (2) for the sake of argument. That leaves (1) and (4) to criticize…