Skip the chapters one and two, if you feel you know the basics of truth theories and wish to see what I want to say about Heidegger.

Most discussions on truth revolve around the same theories: correspondence, coherence, pragmatism and deflationism (and redundancy, if that can be considered as a theory). The simplified definitions of these theories are as follows:

There are huge problems with these. Normally correspondence theory is considered as the "default" theory of truth. That has much to do with the fact that it has been the only truth theory out there for 2000 years or more. It can be retraced back to the Ancients, and often is anchored in Aristotle. This is an important point to note, because it is basically the only reason why one would advocate the other theories of truth (them being rather counterintuitive to begin with). Simply: more and more philosophers conclude that correspondence theory simply doesn't do the job. Despite its initial appeal (that carried it through more than two millennia).

Correspondence theory has a flaw, and that flaw is skepticism. It hands the skeptic all the weapons he would ever need. Because it defines truth as a relation between a proposition and a mind-independent reality, it constructs a relation whose second member is either impossible to reach, or at least exceedingly difficult. Correspondence theory, or so it is often said, actually falls victim to the "certain knowledge" and "knowledge" -dichotomy. It can only establish knowledge in the form of certain knowledge. (Which is not true of other theories.)

Why? Because the relation it fixes is between something mind-dependent and something mind-independent. This constructs a wall between us and the state of affairs. The only way to circumvent this wall is to establish something that simply cannot be false. This, I think, is the basic reason why so many philosophers tried to establish certainty - and when their efforts were considered to be in vain, the new theories of truth arose in the late 19th century.

Let me give an analogy that I have given elsewhere before:
I stand before a wall and I have before me two balls, green and red. Let us also assume that behind the wall there is another like me, holding either the green ball or the red ball in his hand. The wall is solid and I cannot get past it - the other side is beyond my reach. I now hold either of the balls in my hand and state "the ball I am holding is the same ball as the one on the other side is holding". Now we CAN say that my claim is either true or false, given the circumstances. But we cannot say which!

This is the basic problem of correspondence theory: it does establish something that sounds like truth to us, it can even work (that is, we can assume that indeed, our propositions either are true or they are false), but its weakness is in the fact that it seems to rob us of knowledge. This is the reason why other theories have arosen: to save knowledge. This I think is a direct consequence of the gradual change that took place after Kant and because of Kant: the turn from metaphysics to epistemology. That allowed us to see that perhaps fixing truth before knowledge is not such a good idea, and that truth should instead be fixed through knowledge.

A sharp-eyed reader has noted that I skittle around the problem of realism and idealism. I wish to stress that I do not want to enter into a debate about the correctedness of correspondence theory as such, but try to explain what the problems associated with it are and how it gives rise to other theories.

So we come to the primacy of epistemology, and fall into a quagmire.

The pragmatists take truth to be something that is in relation to our needs and their fulfilment. Truth is known by simple measures: if we want to know whether "p" is true, we just have to see if what it means is something that works. Of course, this also leads to relativity of truth, and to the fact that "p" can be true at t-0 and false at t-1 (and true again etc).

The coherentists simply want a system that is not contradictory. Truth is the property of a whole (at least according to the more plausible coherentist-theories, the wholistic coherentism), something that arises from a system of propositions instead of individual propositions, much like temperature in physics. Simply: a proposition by its own has no truth-value, only in relation to other propositions. This of course, even though it allows a neat way of seeing whether our propositions are true or not, seems to lead to a detachment from reality (unlike pragmatism). It also seems to have no criterion as to which one of the coherent systems of propositions is to be assumed. Some theorists combine pragmatism and coherentism to form a rather powerful combination, where the relativity and untrustworthyness of pragmatism is kept at bay by the demands of coherence, and pragmatism then gives means to choose between the coherent systems, and a way to enter them.

Deflationism more or less ducks the problem. It gives formal conditions for a statement to be be true. It is sometimes referred to as a sort of reformulated correspondence theory, because it turns truth into a correspondence between (at least in Tarskian formulation) a proposition of an object-language "p" and the proposition of a meta-language p. Often expressed like this: "Snow is white" is true iff snow is white. The idea is made explicit, when I write: "Lumi on valkoista" is true iff snow is white - where "Lumi on valkoista" is the Finnish expression for "Snow is white".

Abandoning correspondence theory, no matter how compelling it would seem, leaves us in a difficult position considering truth. For correspondence theory truth is something that is, something we can find, and therefore something we can have a correct or false theory about. The epistemological theories give far less value to truth, and even make it completely omittable. Let's go through them one by one again.

Pragmatism takes truth to be functionality, or usefulness. So, what is the position of truth? Basically: "p works" --> "p is true". We could say that truth is the property of every useful proposition, in that if it is useful, then it is true. The implication leaves us asking: why speak of truth at all? Can't we just say "p works" instead of "p is true"? I guess we can.

Coherence theory falls into the same pit. "p coheres with a set of propositions A" --> "p is true". Why can't we speak of p's coherency in system A instead of its truth in system A? Moreover, coherence theory seems to lead to the fact that justified beliefs are knowledge. This then leads to the idea that truth is simply the property of a statement that is known. Truth has no criterium-value, but is merely the entailment for a proposition that is known. Can't we just say "p is known" instead of "p is true"? Again, seems so.

Deflationism is a rather different case. I will not concern myself with it here, because it doesn't really give a criterion for truth as much as express the necessary condition a truth theory must fulfil. In any case, it seems to leave the whole matter of truth open, simply reducing it to a relation between languages. Or a proposition and its meaning. I must confess that I am not that knowledgeable in deflationism to go into details anyway, so I will leave this as it is. (Someone who wants to give a short introduction to deflationism may do so.)

We are left with theories of truth that force us to ask what is the point in such a truth at all. Is truth considered in a wholly faulty way? What if we got it wrong in the first place - what if truth is not to be considered as a property a proposition has, but as something else? This gives us the motivation we are often left lacking when someone throws Heidegger in our faces. So now we have it. Enter Heidegger.

In the book "The Essence of Truth" Heidegger starts with a simple proposition:

But this is just an indirect proof, a modus tollendo tollens. We assume the stance of correspondence and follow it through to its end: to its regressio ad absurdum. How Heidegger does this? After short considerations, he states:

What is Heidegger saying here? Well, if we take the definition of correspondence theory, then truth is the property of a proposition which corresponds to an actual state of affairs. Truths are found. There are states of affairs in the world, and we find them. So is correspondence theory true? If we take the stance of a realist who advocates correspondence theory (can't really be other than a realist), then the correspondence theory is something that is found. There are propositions (produced by subjects) and those either succeed or fail in referring to the outside (objective) world. From this, let us consider the two options available:

Both ways, correspondence theory leads into unintelligibility. Heidegger's regression-argument is powerful. But after destruction must follow construction.

Heidegger takes the battle elsewhere. He first makes the observation that in correspondence theory truth is a property of a proposition. This, he thinks, is either an error, or simply does not define truth as a whole.

Heidegger asks: "what is the essence of truth?" This is most probably something that makes the hair rise for an analytic philosopher. Let us not forget that such analyticians as Kripke have been raising the issue of essence back into philosophy. Just because essence sounds mystical we should not be deterred. By essence Heidegger means the "what-being" of something. He starts with simple cases: what is a table? We can point out objects in our life and say: "that is a table" and "that there is a table too" and "that there is not a table, it is a chair". Heidegger says, correctly I think, that in order for us to say "that is a table", we must already know what a table is. When defining "table", we already know what a table is - otherwise we couldn't define it. This "whatness" of the table he calls the essence of the table. The essence here doesn't have even as much mysticism as Kripke's essences, which, after all, are something that stay the same across possible worlds etc. For Heidegger, at least at the moment, essence is simply that which allows us to recognize different things.

The essence of truth? Well, first Heidegger considers the fact that we all know a host of truths: Earth is round, 2 + 5 = 7, the First World War begun in early August 1914, Kant is the author of Kritik der Reinen Vernunft and Quito is the capital of Ecuador. We also know that 3 + 5 = 6 is false, and that Moon is not larger than the Earth. The former are truths, the latter are falsities. So when we set out to define truth, do we not already know what truth is? Just like we know what a table is? Even though we cannot define it properly? Do we not know the essence of truth? But why is it so hard to define truth, if the essence of truth is readily available to us? Heidegger, if you please.

Everyone who has worked with a lot of problems (like mathematicians) knows that sometimes we are simply too fixated with something to see clearly. We must detach ourselves from the problem, take distance, and only then we can again see the big picture. Often a solution presents itself quite naturally with a little distance. Painting a picture is difficult without sometimes taking a distance to the picture, to see it wholly. The land formations are easier to see when one is sufficiently distanced from them. Even the roundness of Earth is (notoriously so) difficult to see without proper distance. This works for theoretical things too: one of the biggest problems we have in our contemporary philosophy (and have always had) is what is consciousness. But what, indeed, would be closest to us than our consciousness? We simply cannot detach ourselves from it well enough to understand it. And this, my patient reader, is the point of Heidegger here: truth is too close to us, too everyday, to fundamental. He is not saying that truth is something that cannot be grasped, but that we must understand why indeed whatever route we seem to take, we always end up in either answering to a different question, or in the unintelligibility of the answer.

But how to take distance from truth?

Heidegger has a solution: this has already been done by Plato. Heidegger then uses about 100 pages to embark upon one of the most detailed analysis of the Cave-Allegory ever produced. Of course, he does this in such a detail for a proper reason, and a referate of it here would only serve to undermine the point in that analysis. I can say, however, that truth is always present in a way or another. There is always something unhidden for us, but there is pretty much always also something hidden for us. I will give a brief explanation of the concepts hiddenness and unhiddenness.

For Heidegger that which is presented before us is unhidden. That which is not seen or understood by us is hidden. Consider the first step of the Cave-Allegory, where the people are shackled in the cave facing the wall and the shadows. What is unhidden to them are the shadows (note that they wouldn't understand that they are shadows, of course), and what is, at least, hidden to them is the fire that produces the shadows. That hidden can be found. Truth, the unhiddenness of beings, is then something that is grasped through a process of finding things formerly hidden from us. It is very important to understand that "unhiddenness" implies that something must first be hidden, and then uncovered. That is, Heidegger is a realist!

This is, of course, rather vague. Let me try and explain what this might mean - even though I do not consider myself to be in a position to say that how this really is.

For Heidegger truth is not something that is attached to properties. Or, it is, if we take it to be so, but this truth demands something more fundamental to back it up - it is a special case of truth, so to speak. Truth is something that is present to us at all times. The propositions refer to this unhiddenness, and to that we compare the statements in the correspondence theory. A statement that we do not yet know to be either true or false is a statement that lacks the corresponding unhiddenness. "What causes the activity in the wall of the cave" is a question to which we can give answers, but we can know which of these is true only by turning away from the wall, to the fire.

Truth is a property more fitting for experience than for knowledge.

Perhaps this is a way to go? Perhaps what we learn from other truth theories is that truth has no place within knowledge. That truth is something else. Truth is something in the way we experience things - and experiencing is surely a prerequisite for knowing (about the world, excluding analyticity). Perhaps truth is actually something in the belief-requirement of knowledge - in the exact opposite of where the analytic tradition would have it? Perhaps truth is in the vividness of our experiences. Perhaps it is that which causes us to be interested in knowledge in the first place.

Defending Heidegger is too laboursome a task to do here. If someone offers 100 pages of allegorial analysis to back up what he means, it cannot be reduced to few pages of text in an internet forum. I am simply trying to show from what context does Heidegger's idea rise, and show that his ideas are very actual, and should not be ignored. He offers a genuinely different solution to the problem of truth, something that is very near to what analytical philosophy has found. Only that analytic philosophy has not really even started to examine truth in any other way - the closest theories are the redundancy theories, which omit truth as a property of propositions altogether.

I will, in the end, quote myself from an IM-exchange between Valens that occurred some minutes ago:
If it turns out that the red ball you found doesn't fit the square hole, do not throw away the ball, but seek a round hole instead.

What the...? Well: if truth as a property of propositions simply doesn't fit, don't throw away truth: find another use for it.

Source: Heidegger, Martin - The Essence of Truth, Translated by Ted Sandler, Continuum: New York, 2002. Original work Vom Wesen der Wahrheit, 1988.

I recommend the book with all my heart for those interested. It is very approachable and understandable.

Plato's Cave Allegory can be found in The Republic, book VII. An online source.