Let's begin with a familiar problem: the problem of knowledge. This is a central problem for Heidegger.  The problem is to delineate a domain where knowledge can be certain and to determine the criteria for something to fall within this domain of knowledge. 

But as simple as this problem may appear, a little reflection shows much conceptual difficulty. The idea that knowledge needs a criterion presupposes that what we know is not entirely the same thing as "the truth."  So we might ask, "How does knowledge correspond to truth?" 
Or, perhaps, how does the human subject embrace the object of its knowledge? 

First, what is the subject that has knowledge?  Modern philosophy says the subject that knows is a self-aware mind, (the cogito as discussed by Descartes.)   Self-awareness opens up the mind's passage into that which lies beyond itself, into the transcendental realm of that which exists in the world. 

But how are we to understand the mind taking leave of itself and embracing that which exists outside of itself?  Perhaps we should start by noting that the mind does not actually leave itself since the objects it entertains are within it. 

In order to make sense of this paradox, Descartes had to hypothesize the existence of a true god who guaranteed the correspondence between things and ideas.  Furthermore, Descartes had to speculate about the way in which we know truth through reflection (things seem true or false in reflection). But, drawing this conclusion, we are well on the path to solipsism.  In such solipsism the thinking mind does not need to reunite with all that is outside it, for all is within it from the beginning, external object and also the criteria for recognizing its truth.  The subject itself constitutes its own object. It is here that we have the passage into idealism's profound subjectivity.  But one ignores the temporal dimension in order to think of subjectivity as all there is. 

However, with Heidegger we admit that the subject unfolds in time in a chain of causes and effects, and once we do this, everything is not merely mind.  Then, however, we introduce the question as to whether that subject can be called "substance." 

If we say that the subject is substance, then how are we to understand that consciousness is related to each moment of the passing object but not to something which exists permanently?  That is, how can we know the instantaneous without knowing the transcendent which is eternal? 

So, this question about the substance of human subjectivity leads us into a deep negation and paradox.  It is also why the subject who knows must seek to understand the temporal sense of the transcendent object. It is because of our distinctive human subjectivity that Heidegger refers to a human consciousness as a Dasein, which means "being right there" in time. 

Ancient philosophy knew nothing of this modern notion of the subject and subjectivity, of Dasein.  Ancient philosophy never pondered how the temporal way that minds related to that which was external to mind.   All the conceptual difficulties that Plato encountered in the Theaetetus in explaining the possibility of human error originate from his inability to form a notion of the subject and subjectivity.  For him, an error free understanding would rise above the defective temporal understanding. 
 
 
 
 

In contrast to Plato, Heidegger's purpose is to grasp the subjectivity of the knowing person. 
This means determining the way in which time colors our understanding.  For example, consider the understanding we have of the many things in the world that we use as tools, and notice that we  cannot understand tools without handling tools, gaining access to them.  For Heidegger, we can only represent the handled as a thing after we have handled it.  The tool is understood in terms of its function.  The shoe exists in order to be worn.  Thus our understanding of the thing of tools must be founded in a temporal understanding. 

Heidegger's approach creates the concept of an "inner world" without creating a solipsism.  No longer do we think of humans as a consciousness of objects but, instead, as a subjectivity composed of possibilities in time.  "To exist for man is to seize his possibilities.  It is to be his possibilities, which simply means 'to understand them.'" 

Although all our understanding exists within time, there are several modes of understanding within Heidegger.  Although Dasein is ordinarily riveted to its possibilities when it abandons its dream of possibilities it exists in Dereliction.  In dereliction Dasein still feels  but as a letting go of possibility.  Alternatively, Dasein can exist beyond itself, understanding itself as a project-in-draft.  Still another mode is for Dasein to understands itself from the possibility of relating to tools, from entities within the world and not the world itself.  This is the phenomenon of the "fall." 

Dasein's understanding after the fall is worth some reflection.  With the fall, we detach from moral or theological recollection, shunning authentic existence and relapsing into everydayness, understanding ourselves not in terms of authentic possibilities but in terms of the objects we handle.  We become only what we do.  Our relations with others is merely superficial social relations entirely determined by things.  In this state, we feel optimistic but this optimism is a flight in the face of anguish, a flight in the face of authentic understanding of our own mortality. Our lives become filled with chatter and verbiage.   But, ironically, in order to experience this fall, Dasien must first authentically possess itself. 

But however Dasein understands itself, its understanding, and the temporal aspect of its understanding,  is the core of its existence. 

Heidegger calls these fundamental ways of being "existentials".  These distinctions are not trivial because each existential brings with it an affective disposition that shapes our understanding.  For example, one of these moods is anguish.  Anguish brings Dasein back into the world.  Here Dasein is ahead of itself relating to its own termination. 

But regardless of our mode of understanding, the foundational elements of our understanding is always the temporalization of being.  Other things exist without this awareness of time that so deeply characterizes Dasein.  Heidegger calls the existence of such things "presence."  Objects have presence but we only understand these objects within our unique kind of human understanding and subjectivity, an understanding that is profoundly stamped with time.