The question of a priori knowledge with its
For us living in this age, knowledge
amounts to an evidence-based (empirical), effective (esp.
predictive) knowledge for us as subjects of things as
objects, where objects are "things thrown before or presented to the
mind or thought" (Duns Scotus cited in OED). The as here is the
hermeneutic As that shapes how phenomena present themselves to the mind to be understood as such-and-such. Our knowledge of the world in this age therefore depends
upon objects presenting themselves to subjective consciousness
through the sense-data given and is therefore relative to the data
(lit. the given). Kant, along with Descartes, is perhaps the
main messenger for this ontological preconception of knowledge,
which is logical, governed as it is by the rules of
understanding (that include, in particular, the relational rule
of effective causality for relations between objects).
Hegel set himself the grand task of overcoming and 'elevating' this Kantian relativity into absolute (i.e. non-relative) knowing, whose bearer is absolute Geist (mind), the fluid, dialectical movement of the concept in its independence. Geist is able to overcome the relativity of objects being given to consciousness by elevating (aufheben) objective knowledge into the Geist's absolute knowing of itself! It takes up, or reflects, consciousness' knowledge of objects in the external world into its interior, and in so doing absolves itself of its dependency on the givenness of the object via sense data. Hegel thus maintains the inside/outside split of subject/object metaphysics that still reigns today in all science, but the absolute Geist is able to straddle the dichotomy.
Kant himself insists that all knowledge depends upon empirically given experience (sensuous data). Insofar he is an empiricist. But he also shows how the empirically given, sensuous representations (Vorstellungen) are worked up within subjective consciousness under the logical rules of understanding into objects (Gegenstände) that stand over against the subject. Hence he can say famously that the conditions of possibility of experience are the conditions of possibility of the objects of experience. Insofar he offers a genuine subjectivist ontology that lies deeper than any empiricism to which the Anglo-Saxon mind-set succumbed long ago, cutting off any access to deeper questioning of an ontological nature. Hence, in particular, empiricism knows nothing of an a priori knowing whilst at the same time being mired in ontological preconceptions of which it is ignorant as such.
I interpret Hegel's dialectical movement of the concept as the Geist's knowing of its own ontological cast of an historical world. The absolute concept moves fluidly and dialectically entirely within its own element of thinking, thus maintaining its absolute independence from any relativity. Hegel's version of this ontological pre-casting of the world is his Logik, in which he unfolds, in a connected dialectical thinking-through, how the world is cast conceptually in a scaffolding of interconnected categories. This represents an overcoming of the Kantian Ding an sich that Kant claims is unknowable because it is beyond empirically given experience. Geist, by contrast, is able to unfold the a priori (pre-empirical) knowing of the world, its ontological structure in its various categories, by thinking itself (its self).
Hence Hegel represents the
historical culmination of metaphysics as theo-onto-logy and his
philosophy could be called Being and Logos, whereas Heidegger's
is Being and Time. Hegel's categorial casting of world remains true to the logos and therefore is properly called ontology, and theo-ontology due to the absoluteness of Geist. A casting of the hermeneutic structure of the world from ecstatic, three-dimensional time, however, is no longer logical, i.e. based on the logos. Whereas for Hegel the essence (Wesen = whatness) of being itself resides in the logos, for Heidegger, the essencing (Wesen in the verbal sense) of being itself resides in and comprises presencing and absencing, that is, it resides in the openness of three-dimensional time. Beings themselves are recast as presents and absents essencing within this 3D-temporal openness. Instead of being aptly called ontology, this kind of thinking could be named "phenomenophasis", literally, the saying of the phenomena, a term employed in one of Heidegger's very late writings.
To return to ontological preconceptions, an even more basic and ancient one is how we today, unthinkingly and as a matter of course, conceive of the human being as a species of animal. This is a fateful heritage from the Greeks, who cast the human being as the _zoion logon echon_, literally, as the animal that has the logos (language, reason). In the modern age this cast of human being has been modified hermeneutically to the animal that is endowed with cogitating consciousness. (And consciousness, it is postulated dogmatically, is generated by complex neural motions in the material brain.) The human being is thus, paradoxically, a subjective what among objective whats in the world that can be studied by empirical science with a view to understanding and manipulating its movements, its behaviour. Hence modern psychology, the empirical science of subjective consciousness conceived within whatness, which has nothing whatever to do with Aristotle's psychology in his De Anima, which is an ontology of living beings in general.
It is part of our shared historical destiny to understand ourselves as a kind of animal in an evolutionary continuity and discontinuity with 'lower' species of animal, thus as the pinnacle of animal evolution attained over long periods of linear time. This kind of thinking is ubiquitous today and is easy to understand, which accounts for why there is so much talk of evolutionary biology in the media. It's just all so self-evident, and Darwin's evolutionary theory reigns supreme as (empirically) indubitable, which is, indeed, factually correct. But is it the truth of human being itself? In contrast to this casting of human being as a kind of whatness (an animal distinguished or marked by a specific difference), human being itself has to be recast genuinely as whoness from the ground up (from three-dimensional time), and not merely as an ethical afterthought (as it is with Kant, who adds on human dignity to subjective rationality in his Critique of Practical Reason). No wonder Kant is still so well loved today, whereas Hegel is dismissed as a mystical, incomprehensible thinker. This, too, is part of historical human destiny, to live in the untruth of the world. What if, in truth, human being itself belonged first and foremost (a priori, so to speak) to the openness of three-dimensional time?
In view of the hegemonic hermeneutic casting of the human being as a kind of animal, we have the great concern today with the survival of the human species on planet Earth due to environmental degradation and destruction, as if this were the ultimate question confronting us. It is based on a fundamental misunderstanding of who we are because there is no one who risks posing the question of whoness. The question itself is incompatible with the preconceptions of subject/object metaphysics. Thus humanity battles on, in endless power struggles, in coming to terms with the devastation of the Earth that we have wrought, hermeneutically cast as we are as a species of animal (today caught up, in particular, in the gainful game and immersed in the ubiquitous medium of thingified value).
There is an alternative way of thinking (I have provided hints above) articulated already a century ago that is today almost entirely ignored and actively suppressed, by so-called 'professional', academic philosophers in particular, to whom the empiricist-positivist mind-set has assigned the task of administering the end of philosophy.
Further reading: Martin Heidegger Hegels Phänomenologie des Geistes Freiburger Vorlesung WS 1930/31 GA32