A half-truth is a deceptive statement that includes some element of truth. The statement might be partly true, the statement may be totally true, but only part of the whole truth, or it may use some deceptive element, such as improper punctuation, or double meaning, especially if the intent is to deceive, evade, blame or misrepresent the truth.
The purpose and or consequence of a half-truth is to make something that is really only a belief appear to be knowledge, or a truthful statement to represent the whole truth or possibly lead to a false conclusion. The order in which the true and false information is presented in a "half-truth" can make a difference in ultimate believability. Barchetti and colleagues show that when two unrelated statements are but together with syntax that suggests causality, the statement is believed if the premise is true (even if the conclusion is unrelated or false). Conversely, if the false statement is placed in the premise, the combined statement is less likely to be believed. Thus order of presentation can influence the credibility of a half-true statement and has been named the half-truth effect. According to the justified true belief theory of knowledge, in order to know that a given proposition is true, one must not only believe in the relevant true proposition, but one must also have a good reason for doing so. A half-truth deceives the recipient by presenting something believable and using those aspects of the statement that can be shown to be true as a good reason to believe the statement is true in its entirety, or that the statement represents the whole truth. A person deceived by a half-truth considers the proposition to be knowledge and acts accordingly.
Some forms of half-truths are an inescapable part of politics in representative democracies. The reputation of a political candidate can be irreparably damaged if they are exposed in a lie, so a complex style of language has evolved to minimise the chance of this happening. If someone has not said something, they cannot be accused of lying. As a consequence, politics has become a world where half-truths are expected, and political statements are rarely accepted at face value.
William Safire defines a half-truth, for political purposes, as "a statement accurate enough to require an explanation; and the longer the explanation, the more likely a public reaction of half-belief".
It has been shown that the order of the half-truth makes a difference in reported belief in the statement. That is when a statement begins with a true statement followed by another unrelated statement (either true or false), the statement is believed. However, when the false statement is put in front, then the entire package is less believed regardless if the second part of the argument is true or false.
In his 1990 work The Magic Lantern: The Revolution of 1989 Witnessed in Warsaw, Budapest, Berlin, and Prague, Timothy Garton Ash responded to Václav Havel's call for "living in truth":
Now we expect many things of politicians in a well-functioning parliamentary democracy. But "living in truth" is not one of them. In fact the essence of democratic politics might rather be described as "working in half-truth". Parliamentary democracy is, at its heart, a system of limited adversarial mendacity, in which each party attempts to present part of the truth as if it were the whole.
Philosopher Alfred North Whitehead was quoted as saying: "There are no whole truths; all truths are half-truths. It is trying to treat them as whole truths that play the devil". If this is true, statements, or truths, which according to Whitehead are all half-truths, are susceptible to creating deceptive and false conclusions.
Richard Brodie links half-truths to memes, writing, "the truth of any proposition depends on the assumptions you make in considering it—the distinct memes you use in thinking about it". Brodie considers half-truths a necessary part of human interaction because they allow practical application of ideas when it is impractical to convey all the information needed to make a fully informed decision, although some half-truths can lead to false conclusions or inferences in the world of logic.
The notion of half-truths has existed in various cultures, giving rise to several epigrammatic sayings.