The title says it all. While I have not studied it, I have repeatedly heard that lojban is an especially "logical" language, one that could even be a way to speak a "natural" language to the computer that it can understand.

What feature of lojban (or features) makes this so? Is it just that humans haven't gotten to it yet?

  • 2
    Yes - would it remain so pure if a speech community used it like any natural language? I doubt it.
    – curiousdannii
    Feb 7, 2018 at 1:17

3 Answers 3


Lojban was designed with a few key features in mind, which are ultimately why it claims the title of "logical" language.

First it's good to know that a logical language actually means something pretty specific. It's usually built on predicate logic, and a desire to express any logically-expressible statement unambiguously. No two statements are allowed to mean the same thing, and no statement is allowed to mean two things.

In this sense, Lojban fulfills the literal criteria for what counts as a logical language. There's also the colloquial sense, though - the way people think about it, when someone says "logical language."

It is designed to be a regular language. This gives it a few key properties that make it easier for computers to process (though it's still an immensely complicated task to implement). Most of these properties are mathematical, and you can read them on the Wikipedia page. However, what it means in an intuitive way is that no word or word phrase's meaning changes based on words it's not adjacent to. That's a nice property when you're a computer and have a hard time maintaining a sense of context.

Lojban also has no irregular forms or rule exceptions whatsoever. All words are constructed based on specific properties that are clearly delineated, using roots that are unique and are intended to mesh with each other as-is.

This means that any construction which can be built in a logical form can be written completely and uniquely in Lojban. There are no ambiguities between forms. Note, though, that this is commonly confused with "the language has no ambiguity." It's possible to be ambiguous in Lojban, but only intentionally, and that ambiguity is unique to the way you wrote it.

It also allows for systematic, structured learning and use. New words fit into the existing grammar and structure without any other prompting, once you know what they mean and how they're used. No additional grammatical structures are needed to use any words, once you know them.

  • 1
    A regular language is a set of strings definable by some regular expression or (equivalently) some deterministic finite automaton. Lojban is not.
    – Jordan
    Jun 11, 2021 at 22:46
  • Actually, many Lojban utterances are equivalent to each other; for example, CLL claims {mi prami do} and {do se prami mi} are equivalent. This sort of equivalence is essential to logic in practice, since a deductive system with no equivalences has no interesting tautologies, let alone any interesting deductions!
    – Corbin
    Jul 11, 2023 at 1:56

What is meant by "logical language", is that the language's grammar is engineered so that any statement you make is syntactically unambiguous. Grammatically correct statements only parse one way.

In natural languages, and many constructed languages, the meaning of a statement can be interpreted to be a number of things, depending on context.

An example to given in the "Complete Lojban Language" book (CLL), is the term "Pretty Little Girls’ School". In English, there's no way to tell what this means. Does it mean a school for "pretty little girls", a pretty school for "little girls", or a small school that is pretty and for girls?

Lojban's grammar is such that the relationship between words is unambiguous. The meaning to words themselves doesn't matter all too much to the language's designers, just the relation between those words. As such, ambiguity is still possible, but usually in the form of semantic vagueness.

That said, it's not like there's only one way to say anything. According to the CLL, there are Forty ways to translate the English phrase "Pretty Little Girls' School" into Lojban. Among them, is melbi cmalu nixli ckule, which would be a direct translation of the English phrase, word-for-word, and refer a "school for girls who are beautifully small". Whatever "beautifully small" means.
But if you go through the list of possible interpretations, you'll notice that there are in fact some statements that evaluate to the same meaning, either because that's how the semantics work out, or because the two statements can be logically equivalent. (2 + 2 = 4 is equivalent to 2 × 2 = 4 and 2² = 4)

Ultimately, these properties are what make Lojban a tempting tool to use for communicating with AI, who are notoriously bad at working out vague context we use in everyday speech. And it's also why Lojban is called a "Logical Language".

  • Hm, I remember seeing an appendix to one of the old Loglan books that listed seventeen meanings for “pretty little girls' school”. Now there's forty? Progress! Oct 9, 2018 at 3:00

The syntax of a logical language is amenable to rewriting, which generates a deductive system, a special case of a formal system. There are two main features of a logical language:

  • Some utterances are provably equivalent to each other
  • Some utterances provably imply other utterances

For the case of Lojban, I maintain brismu, a living collection of rewrite rules and formal proofs.

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