As the above sentence - in its entirety, so all that is italicized - must be true, it forms the proof of the existence of absolute truth.
NOTE: In the light of incomplete, erroneous and wrong quotations of this proof by people in discussion fora, guest books and the like, I stress it is forbidden to reproduce this proof in altered or incomplete form. To alter someone's words (including leaving things out) is copyright violation. If you are unable or unwilling to quote properly, rather refer to this web page and abstain from quotation.
The adjective "absolute" in this proof should be taken in its literal sense, so as "the opposite of relative", or as "objective". It must not be confused or related to "divine" or "infinite". I state this because I have noted some attach those meanings to "absolute", and even use the symbol ∞ to refer to "truth" as meant here. But this proof has nothing to do with God or infinity. I have considered removing the word "absolute" from the proof (without altering the proof's meaning in any way, as "absolute truth" simply means the same as "truth") to avoid this confusion, but have not decided about that yet. After all, it is the right word for the meaning intended, and since many people use "truth" in a relative, subjective sense ("One person's truth is not the same as another person's truth"), the proof is clearer to those people this way, despite the redundancy of "absolute".
The adverb "absolutely" in the proof should taken as modifying "true", not as modifying the "is" that precedes it.
What is the absolute truth then? The proof speaks of "absolute truth", not "the". It is not a particular truth, but the general concept of things being able to be objectively true, as opposed to all things being relative, subjective and uncertain.
The proof only shows that the sentence itself is true, and says nothing of other truths. The fact that there is one true sentence is proof that absolute truth exists; only one instance is needed to prove that. If at least one sentence is true, we know it is possible for sentences (statements, propositions, claims, assertions) to be true. In other words, the "null hypothesis" (absolute truth does not exist) has been rejected. Note well: To say that it is possible for sentences (statements, et cetera) to be true in no way implies that there actually exist multiple true sentences!
The sentence contains self-reference. It is a fallacy to think self-reference makes a proof invalid.
The claim "Absolute truth does not exist" is shown to be not absolutely true, but that doesn't prevent it from being relatively true. This comment would be true if relative truth would exist. But it does not; things are true or not true, there is nothing in between. If there was, one could "prove" or "disprove" anything, and it would mean nothing. To assume the existence of relative truth is a bit like saying, "Everyone can be a little bit right, in his own way". For words to be used for what is mistakenly called "relative truth", see the response to the comment "Truth is subjective, one person's truth is not the same as another person's truth".
Truth is subjective, one person's truth is not the same as another person's truth. Words like "truth" and "true" are reserved for information that is independent of individual subjective perception, objective, factual, proven, absolute. For information that is dependent on individual perception, subjective, opinionative, suspected, relative - so, for information that differs between subjects - use words like "opinion", "view", or "perception". For instance, do not say "One person's truth is not the same as another person's truth", or "Whose truth do you mean?". Instead, say "One person's opinion is not the same as another person's opinion", or "Whose opinion do you mean?", because that is what you mean to say.
1 + 1 = 2 or 1 + 1 = 10 ? What is true? "1 + 1 = 2 in the decimal system" is true, and that is what you implicitly mean when you say "1 + 1 = 2". That other number systems exist does not make that any less true. You always take the system into account, explicitly or implicitly. Also, 10 in the binary system has the same meaning as 2 in the decimal system, so in terms of meanings - which is what counts - both say the same anyway. This comment merely plays with different notations for the same meaning, and says nothing about the meaning itself.
This example can be taken to a more general level by pointing out that a sentence or statement must be fully qualified in order to assess its truth value. And indeed it can be very hard to formulate, define, a fully qualified sentence that can be deemed true with certainty; but that fact does not contradict the proof, as only one true sentence is required to prove that it is possible for sentences to be true.
Newton's laws were thought be true, but corrected by Einstein's theory. And even that cannot be proven to be absolutely true. They are each only true within their own context or system. Nothing can be absolutely true. The latter is obviously an absolute statement (and therefore contradicts itself), so who says that does believe in absolute truth after all, but does not realize it yet. This person is caught in a vicious circle of recursive reasoning, the only way out of which is to accept things can be absolutely true (which one obviously already believed in the first place, as betrayed by that last sentence). It is amazing people can be so blind to their own error; they say "Nothing can be absolutely true", and still do not "get" it. And in case one still does not "get" it after reading this paragraph up to here: If "Nothing can be absolutely true", then "Nothing can be absolutely true" can not be absolutely true either, so that one is not saying anything at all but merely undermining one's own statement. To understand this, take a step back, "step out of the system", or "out of the box" if you prefer, widen your associative horizon (if you can), curl back on yourself, jump over your own shadow, and oversee the whole - including yourself - of what is going on.
Does this mean that you know the answers to all questions? No, because (1) the fact that absolute truth exists does not imply that the answers to all questions are or can be known, and (2) the fact that absolute truth exists is a general fact, and is not connected to any particular individual, such as the one who has formulated the proof on this page. Do note that the answer in this paragraph in no way excludes the possibility that a particular individual knows the answers to all questions, but merely explains that the proof does not imply that a particular individual knows the answers to all questions.
Over the years that this proof has been published I have learnt something about the personality of those who can not understand it, who object to it: They appear to have narrow associative horizons, even when otherwise intelligent and conscientious. Perhaps to fully understand the self-reference involved, a wide associative horizon is required, and therefore philosophical relativism ("Truth is subjective") may result from narrow associative horizon combined with high intelligence and conscientiousness. This is in line with a more general hypothesis that has occurred to me over the past years: That Marxism, which is the most general term I have found for the whole of the many forms of relativism and truth-denial, is rooted in this same personality type: "Intellectuals" is a term that seems to describe them. Some object to the term "Marxism" saying it has only to do with economy; alternative terms one may prefer are neo-Marxism, cultural Marxism, egalitarianism, leftism, and leftist intellectualism.
While a personality profile may cause or dispose for inability to accept the proof, interest may help to sustain their denial of it. Who have the greatest interest in denying that things can be objectively true? Obviously, those who believe in ideas that are not true; philosophical relativism is what protects their ideology from being exposed as false, for if nothing can be objectively true, nothing can be objectively false either. They must keep up the mantra of "Truth is subjective", otherwise their belief system and their world will fall apart. When I first formulated this proof years ago, I was naive and had no idea of these ideological connotations. Only truth mattered to me, and I had and have no "interests" in things being one way or the other. But the political-ideological interests of the deniers of the proof have become apparent to me as time went by.