Editing Karl Popper (section)

==== Truth ====
As early as 1934, Popper wrote of the search for truth as "one of the strongest motives for scientific discovery."<ref>{{Cite news |last=Williams |first=Liz |date=10 September 2012 |title=Karl Popper, the enemy of certainty, part 1: a rejection of empiricism |work=The Guardian |url=https://www.theguardian.com/commentisfree/2012/sep/10/karl-popper-enemy-uncertainty |access-date=22 February 2014}}</ref> Still, he describes in ''Objective Knowledge'' (1972) early concerns about the much-criticised notion of [[Correspondence theory of truth|truth as correspondence]]. Then came the [[semantic theory of truth]] formulated by the logician [[Alfred Tarski]] and published in 1933. Popper wrote of learning in 1935 of the consequences of Tarski's theory, to his intense joy. The theory met critical objections to [[truth]] as correspondence and thereby rehabilitated it. The theory also seemed, in Popper's eyes, to support [[metaphysical realism]] and the regulative idea of a search for truth.

According to this theory, the conditions for the truth of a sentence as well as the sentences themselves are part of a [[metalanguage]]. So, for example, the sentence "Snow is white" is true if and only if snow is white. Although many philosophers have interpreted, and continue to interpret, Tarski's theory as a [[Deflationary theory of truth|deflationary theory]], Popper refers to it as a theory in which "is true" is replaced with "[[correspondence theory|corresponds to the facts]]". He bases this interpretation on the fact that examples such as the one described above refer to two things: assertions and the facts to which they refer. He identifies Tarski's formulation of the truth conditions of sentences as the introduction of a "metalinguistic predicate" and distinguishes the following cases:

# "John called" is true.
# "It is true that John called."

The first case belongs to the metalanguage whereas the second is more likely to belong to the object language. Hence, "it is true that" possesses the logical status of a redundancy. "Is true", on the other hand, is a predicate necessary for making general observations such as "John was telling the truth about Phillip."

Upon this basis, along with that of the logical content of assertions (where logical content is inversely proportional to probability), Popper went on to develop his important notion of [[verisimilitude]] or "truthlikeness". The intuitive idea behind verisimilitude is that the assertions or hypotheses of scientific theories can be objectively measured with respect to the amount of truth and falsity that they imply. And, in this way, one theory can be evaluated as more or less true than another on a quantitative basis which, Popper emphasises forcefully, has nothing to do with "subjective probabilities" or other merely "epistemic" considerations.

The simplest mathematical formulation that Popper gives of this concept can be found in the tenth chapter of ''Conjectures and Refutations''. Here he defines it as:

: <math>\mathit{Vs}(a)=\mathit{CT}_v(a)-\mathit{CT}_f(a) \,</math>

where <math>\mathit{Vs}(a)</math> is the verisimilitude of ''a'', <math>\mathit{CT}_v(a)</math> is a measure of the content of the truth of ''a'', and <math>\mathit{CT}_f(a)</math> is a measure of the content of the falsity of ''a''.

Popper's original attempt to define not just verisimilitude, but an actual measure of it, turned out to be inadequate. However, it inspired a wealth of new attempts.{{sfn|Thornton|2015}}