Loglan is a constructed language originally designed for linguistic research, particularly for investigating the Sapir-Whorf Hypothesis. The language was developed beginning in 1955 by Dr. James Cooke Brown with the goal of making a language so different from natural languages that people learning it would think in a different way if the hypothesis were true. Loglan is the first among, and the main inspiration for, the languages known as logical languages, which also includes Lojban and Ceqli.
Dr. Brown founded The Loglan Institute to develop the language and other applications of it. He always considered the language an incomplete research project, and although he released many papers about its design, he continued to claim legal restrictions on its use. Because of this, a group of his followers later formed The Logical Language Group to create the language Lojban along the same principles, but with the intention to make it freely available and encourage its use as a real language.
Supporters of Lojban use the term Loglan as a generic term to refer to both their own language, and that of The Loglan Institute. They refer to the latter language as TLI Loglan when in need of disambiguation. Although the non-trademarkability of the term Loglan was eventually upheld by the United States Patent and Trademark Office, many supporters and members of The Loglan Institute find this usage offensive, and reserve Loglan for the TLI version of the language.
Brown intended Loglan to be as culturally neutral as possible, and metaphysically parsimonious, which means that obligatory categories are kept to a minimum. An example of an obligatory category in English is the time-tense of verbs, as it is impossible to express a finite verb without also expressing a tense.
Also, Brown intended the language to be totally regular and unambiguous. In particular, phonemes that could be confused with each other were to be avoided.
The language’s grammar is based on predicate logic, which is why it was named Loglan, an abbreviation for "logical language". This has been thought to make it suitable for human-computer communication, which led Robert A. Heinlein to mention the language in his science fiction novel The Moon Is a Harsh Mistress, and as a fully fledged computer language in The Number of the Beast.
Loglan has no distinction between nouns and verbs. The predicate words can serve as verbs, nouns, adjectives or adverbs depending on where they occur in a sentence. Each predicate has its argument structure with places for arguments, which may be variables. For example: vedma, "X sells Y to P for price Q". Prefixes allow one to reorder the argument structure of predicates, to emphasize one of the variables by putting it first. For example, to make price the first variable, use ju vedma (with the "little word" ju). Similarly, the sentence can be reordered to speak about seller, ware, or buyer. Modifications for time, location, actor, type of action, and others are provided by "little words" which are optional. Predicates can be compounded: a predicate can act as an argument of another predicate, when the former is prefixed by a "little word".
The language is designed so that the patterns of phonemes always parse into words uniquely. Even when run together, the words can be separated in only one way. In Loglan, one can directly and precisely say any of the different meanings of the English phrase "a pretty little girls’ school." This feature is so pronounced that people fluent in Loglan say impossible things as a sort of joke—a type of humor not supported by the linguistic machinery of other languages. In Loglan, it is also possible to say that John, a person, is literally a short word.
Loglan has a wide range of words used for expressing emotions and attitudes about what one is saying, but unlike natural languages, these are kept clearly distinct from the actual statements being made. This may be surprising to people who assume that a language based on logic would be computer-like, and devoid of human emotion.
Loglan was mentioned in a couple of science fiction works: Robert A. Heinlein’s well-known books The Moon Is a Harsh Mistress and The Number of the Beast, and Robert Rimmer’s utopian book Love Me Tomorrow. The narrative of The Moon Is A Harsh Mistress, though written in English, is nominally almost entirely in Loglan as the story is narrated by a character from the lunar colony, where everyone speaks Loglan.
Loglan’s inventor, James Cooke Brown, also wrote a utopian science fiction novel The Troika Incident that uses Loglan phrases but calls the language a different name.
Loglan is used as the official interspecies language in the roleplaying game .