Vancouver Philosopher at The Chasm makes the following point about Heidegger:
Heidegger's denial about a fundamental unifying characteristic to philosophy is foolish. When we employ such skepticism about the reality of the ground/central concept, we only wind up grounding such skepticism in denial. The very act of denial becomes its own ground, and this strategy winds up being self-defeating in the end. Instead, we shouldn't think of various philosophical systems as themselves foolhardy in establishing a ground or framework. We should interrogate the framework or ground on its own merits.
This follows my own thoughts about skepticism regarding truth: we cannot sensibly make a statement like "There is no truth" without simultaneously undermining that statement by implying that it itself is true. For skepticism to assert a meaningful proposition (that is, a proposition which supplies us with genuine information), the skepticism itself must be grounded in something. And if the brand of skepticism at hand denies the existence or reliability of all grounds, it necessarily denies its own conclusions. Hence, as Vancouver Philosopher says, denial becomes its own ground, and it invariably ends up defeating itself.
Similarly, skepticism about our epistemic relation to truth undermines itself as well. Suppose our skeptic offers us "an irrefutable argument" that, whether there exist truths or not, we simply cannot possess knowledge of them. E.g., all claims must be justified by other claims, and there are no unjustified, self-supported claims; ergo all claims are unjustified/unacceptable. And yet, if we accept the skeptic's conclusion, we (presumably) have now acquired a new bit of knowledge, viz., the knowledge that "All knowledge is impossible", or "We possess no knowledge"! Wait, how did that happen? How do we know this when we can't know anything? Again the skeptical conclusion undermines itself; it presupposes its own new ground from which to criticize the whole of another position, yet in doing so creates and relies upon that which it seeks to demolish.
Similar arguments apply to skepticism toward the legitimacy of reason generally.
The essential problem, as I see it, is that the skeptic must engage the opponent on her own territory, so to speak, and using her own tools. This makes strategic sense, since otherwise why should the anti-skeptic (let's say "dogmatist") accept whatever point the skeptic tries to make? Unfortunately, this is a rigged game for the extreme skeptic: when playing by the dogmatist's rules, there is simply no way to "win" or "break outside" the system, because every attempt to do so places you right back into a new system. (This all ties into the Liar's Paradox, Gödel's Incompleteness Theorem, and the Halting Problem; I don't know how to make this more explicit yet, but see Gödel, Escher, Bach by Douglas Hofstadter for related discussion).
Now, life isn't exactly a bed of roses for the dogmatist, either, since she can never really answer the skeptic's demands for an "unmoved mover" in the realm of logic, so to speak. But it's fascinating to imagine the two positions, intertwined together as an infinity of recursive, dialectic contradiction. The skeptic asserts, "You know nothing"; the dogmatist responds, "Then I must know that I know nothing"; the skeptic rejoins, "But to know that, you must presuppose other principles for which you have no justification either!"; and the dogmatist counters, "But in order to know that I need those principles, surely I must know that I need them, so I do know something after all!", and so on ad infinitum. Or ad nauseam, take your pick. The two warring sides endlessly wrap around each other, neither overcoming the other--perhaps like twin strands of intertwined DNA which end up consuming their own tails as an involuted Ouroborus?
But let's not get ahead of ourselves, nor lost in mystical/metaphorical speculation. The key here is that we could apparently resolve the tension if we could ever find a groundless ground. What I call a "groundless ground" shows up in many places: as Aristotle's "prime mover," as Aquinas' "uncaused cause," as the idea of a necessary entity or fact in general, as a self-caused being, as that which needs no justification, as a "primitive fact," as Kant's description of "the unconditioned." To find such a ground and look out from it would yield the fabled view from nowhere, the view sub specie aeternitatis, the God's-eye view. This perspective, and none other, would be satisfactorily "outside of the system" to satisfy both skeptic and dogmatist. (Hopefully.)
It's no surprise, then, that my description of the clash above mirrors Kant's Antinomies of Pure Reason, where my "dogmatism" would map on to the rationalist understanding of metaphysics, and skepticism to the empiricist understanding. Roughly, anyhow. To be more precise, Kant thought that both rationalist and empiricist sought a "groundless ground" (the "unconditioned"), but they sought it from different starting premises; whereas my dogmatist/skeptic divide does not show both sides seeking an unconditioned ground so much as the denial that there is such a thing on the skeptical side. So take the comparison with a grain of salt--I'd have more to say about justifying my analogy/mapping, but I'm running out of motivation at this point.