Logical positivism is a way of thinking that was popular in the mid-20th century, and which attempted to make philosophy more rigorous by creating criteria for evaluating the truth or falsity of certain philosophical statements. Its main criteria for any statement is verifiability, which comes from two different sources: empirical statements, which come from science, and analytic truth, statements that are true or false by definition. The concept heavily influenced the philosophy of science, logic, and the philosophy of language, among other areas, though today it is largely viewed as an overly simplistic approach that has been displaced by newer philosophies.
This philosophy is an absolutist way of looking at statements and labeling them either true, false, or meaningless. In modern times, this has been displaced by philosophies that view the truth or falsity of statements in a probabilistic rather than an absolute light. Logical positivists themselves had many disagreements, demonstrating that this notion was more a cluster of philosophies rather than any monolithic code.
A key component of logical positivism is that it rejected statements about ethics and aesthetics as being unverifiable, and therefore not a part of serious philosophical thinking. To have meaning, a given statement had to be connected to either empirical data or analytic truth. This was a key step in connecting philosophy more closely to science, and vice versa, and it continues to have influence to the present, playing a vital part in the formulation of philosophical ideas throughout the 20th century.
Though logical positivism was originally popular with many philosophers in the Vienna Circle — a group of philosophers instrumental in developing analytic philosophy — it came under fire from many experts after the circle was essentially forced to disband when World War II began. Afterwards, many philosophers criticized the approach, including Bertrand Russell, despite the fact that some of his ideas on logic actually influenced its development. Likewise, though Ludwig Wittgenstein's Tractatus Logico-Philosophicus influenced logical positivists, Wittgenstein himself said that their rejection of certain parts of the Tractatus demonstrated that they had fundamental misunderstandings about the book.
Later thinkers distinguished between two classifications of verifiability: "strong" and "weak" verification, the former being something that is conclusively established by experience, the latter only being rendered probable by experience. Many philosophers have criticized logical positivism for having a "self-application constraint": logical positivists claim that sentences are not verifiable, yet they still posit their theory with sentences. This makes their approach untenable, since they claim a theory to be true, but the theory cannot apply to the sentences that they use to state the theory.