Unless one were to doubt that we know anything at all, we have knowledge of things, and these things exist. Arguably, the first move of idealism in philosophy is to take as paradigmatic this kind of overlap between what we know and what there is. It triggers an enquiry into what it means to know something and what that something, or anything, really, must be like for us to know it. In both epistemology and ontology, asking these sorts of questions is the approach of transcendental philosophy. In Kant's formulation, we ask for the ‘conditions of possibility of the objects of experience’, of those things that exist and that we know.

But things may go wrong, or stall. What if I presume that something exists, like the solution to a problem or the cause of something I see happening – but I cannot bring them clearly into view? In these cases, we know that there is something to be known, but don't know how; we reach a limit of knowledge. And what about the case where something completely unexpected happens? Here, something comes into being from beyond the limits of what we can know. But expected or not, once it is here, it no doubt exists, calling into question the notion that being and knowing are in principle co-extensive. The Kantian idealist's response to these kinds of cases is that they don't really matter. What counts is the good case, where cognition succeeds. Especially its very opposite, the case of something we don't know and can't even anticipate – why care? And how could we even think about that?

Kate Withy's book on the philosophy of Martin Heidegger shows that he did care, and it makes a good case that we should, too. The argument Withy sees Heidegger pursuing is a variation on the transcendental line of questioning, but with a crucial shift from clear success at knowing to limit cases. In fact, Heidegger takes these cases to provide an answer to the ontological question: for something to exist means for it to be, to some extent or in some respect, inaccessible to us. It is precisely the fact that things, to use Heidegger's language, ‘withdraw’ or ‘conceal themselves’ that vouches for their reality.

A first case of concealing is that of the ‘backgrounding’ (§ 7) of the conditions on which I make sense of something. Heidegger approaches this case through the thick descriptions of the exercise of skill that his Being and Time is known for: when I am engaged in a carpentry job, I use the tools and resources around me to achieve my goal. Absorbed in my task, the things around me are immediately meaningful to me, they are ‘discovered’ (Heidegger) in how they are relevant to that task. But this doesn't mean that there are no conditions enabling me to experience things in this way. There is a whole ‘world of carpentry’ defining what things are for and what I can do with them. But as I hammer away, I don't notice: ‘In order to engage with the entity discovered, we overlook or look through the world on the basis of which it was discovered. In this way, discovering any entity conceals the world’ (p. 49). As Withy emphasises, Heidegger in Being and Time presents the good case of ‘skilful coping’ (Hubert Dreyfus) only to contrast it with the many ways things may go wrong: the hammer is too heavy, it is unsuited for my task, or it breaks. In those situations, my sense-making is interrupted, and I can take heed of the conditions I have been relying on. As my attention shifts from the things to the world in which they have meaning, I become aware that the exercise of skill, while revealing things, is itself far from transparent. This is a first form of ‘concealing’.

Another example comes from the use of language. Heidegger in Being and Time also defends the idea that the propositional form of knowledge harbours a similar form of concealing. As a case in point, think of describing a painting. There are many true things you can say about it, but if nothing replaces seeing the real thing, this is not (as some commentators have argued) because there is something so fine-grained in my perception that language cannot properly express it. Rather, as my words discover the painting being a certain way, they ‘cover over’ other ways in which it might be known, other things I could say about it: ‘What it is to uncover an entity as a determinate that and what is to simultaneously conceal other (suitably opposed) thats and whats that it might manifest as […]. The concealment of the entity as y – and as p, q, and r – means that there is a withheld abundance of intelligibility in entities: an abundance of ways in which it might be discovered in our comporting toward them’ (p. 32). As in the case of skilful coping, language here functions as a medium to meaning that, while allowing things to show up a certain way, is itself opaque.

One of the achievements of Withy's book is to directly address, clearly distinguish, and plausibly relate such different phenomena of concealing and their corresponding forms of discovery. Withy develops what she calls a ‘taxonomy’ (§ 2) of different phenomena of epistemic absence, and while this approach is well suited both for an interpretation of Heidegger and for showing the relevance of his work, its first presentation makes for quite a shock. The reader is given three different tables, mostly blank, to be filled over the course of the book. Similarly surprising, but ultimately useful, is her choice to take over in her own presentation some of the ancient Greek terminology Heidegger adopts from the history of philosophy. One of these is the binary of ‘concealing’ and ‘unconcealing’ Heidegger models on the ancient Greek word for truth, alētheia, that combines the word for forgetting or ignorance (lēthē) with a prefix indicating a privation. The lesson Heidegger draws from this exercise in semantics is the primacy of lēthē over alētheia, of ‘concealment’ over ‘unconcealment’, and he thus translates alētheia and the corresponding success verb for knowing something (alētheuein) as ‘unconcealment’ and ‘unconcealing’. Rather than toward the good case of cognitive success, philosophy should be oriented toward the fact that whatever we know is wrested from ignorance. Heidegger speaks of our discovering entities to emphasise this point: coming to know them is an achievement, although it is part of the conditions of this achievement that it requires special attention to notice them.

Using Heidegger's Greek, Withy groups together the backgrounding of the world and the propositional use of language as forms of kruptein, ancient Greek for ‘to hide’ or ‘to conceal’. In these kinds of cases, something conceals something else: skilful agency and propositional language cover over what makes them possible. One of the merits of Being and Time is that it extends the transcendental project to the concealing operative in them – giving an account of lēthē, the black hole of knowledge, is more difficult. Heidegger orients transcendental philosophy most radically away from the good case in his attempts to address this fundamental ignorance: ‘We can glimpse lēthē in the condition of the animal and we can experience our lēthē itself liminally in a mood such as angst. In both cases, however, we can at most only graze this non-intelligibility. […] “Everything disappears” […] in this darkness – or rather, since darkness presupposes light, in this absence of both darkness and light’ (p. 89). Withy links discussions found in different contexts to show that they cohere precisely as treatments of lēthē. But this is not the gist of her reading of Heidegger, as she finds his crucial insight to lie neither in the discovery of kruptein nor in these accounts of fundamental concealment. Heidegger's main achievement is to have identified a different, third type of concealing in his later works.

Heidegger here takes his cue from a fragment of Heraclitus (Fr. 123) according to which physis krupthestai philei, ‘nature loves to hide’. Withy is the first commentator to give a plausible account of why and how this slogan of Presocratic philosophy matters to Heidegger: not because it gives another intimation of lēthē but because it differentiates the middle-voiced krupthestai from kruptein, self-concealing from other-concealing. On this interpretation, Heraclitus’ point is not that we can never know nature as it really is. The fragment rather concerns the process by which the transcendental structure supporting knowledge is itself established or, in Heidegger's terminology, the way in which being becomes manifest, ‘shows itself’. Heidegger's Heraclitean insight is that, on this most basic level of analysis, the enabling (but themselves withdrawing) conditions and what they make possible (and thus reveal) cannot be separated. While the backgrounding of skill and language can be foregrounded and our knowing and doing be made transparent through what Heidegger calls ‘authenticity’ (§ 21), the same is not true for the self-concealing of nature. Because physis is the underlying process that establishes the conditions of each entity and its intelligibility, its self-concealing is a general feature of all being and knowing. It is not a restricted feature of human skill or language that what makes them possible remains opaque; it is the principle of all there is and all we can come to know: ‘In concealing itself, phusis conceals the emerging of entities as a whole and as such into appearing. The entities still emerge into appearing, and their manifestness is not threatened or thwarted by concealing. What is concealed is the event of appearing – the emerging, not what emerges. In this sense, the appearing and the concealing operate at different registers and so do not come into conflict. To return to the metaphor of illumination: krupthestai is not analogous to night, where nothing can be seen, but instead to the daylight that allows things to be seen but is itself hidden. In this sense, light is indeed dark’ (p. 26).

Once all the cells of the tables have been filled, Withy gives us more than a mere taxonomy of different kinds of, say, epistemic negativity in Heidegger. From her reading of the thorniest texts of Heidegger's philosophy emerges a nuanced phenomenology of the limits of knowledge and a coherent account of the alternative to Kantian transcendental philosophy he offers. Because Heidegger ultimately equates phusis with being, Withy can even lay claim to the main prize in the interpretation of this thinker. Her book provides an account of what Heidegger means by being that is both exegetically compelling and descriptively plausible. To be is to be self-concealing.