A philosophical language is any constructed language that is constructed from first principles, like a logical language, but may entail a strong claim of absolute perfection or transcendent or even mystical truth rather than satisfaction of pragmatic goals. Philosophical languages were popular in Early Modern times, partly motivated by the goal of recovering the lost Adamic or Divine language. The term ideal language is sometimes used near-synonymously, though more modern philosophical languages such as Toki Pona are less likely to involve such an exalted claim of perfection. It may be known as a language of pure ideology. The axioms and grammars of the languages together differ from commonly spoken languages today.
In most older philosophical languages, and some newer ones, words are constructed from a limited set of morphemes that are treated as "elemental" or fundamental. "Philosophical language" is sometimes used synonymously with "taxonomic language", though more recently there have been several conlangs constructed on philosophical principles which are not taxonomic. Vocabularies of oligosynthetic languages are made of compound words, which are coined from a small (theoretically minimal) set of morphemes; oligoisolating languages, such as Toki Pona, similarly use a limited set of root words but produce phrases which remain series of distinct words.
Toki Pona is based on minimalistic simplicity, incorporating elements of Taoism. Láadan is designed to lexicalize and grammaticalize the concepts and distinctions important to women, based on muted group theory.
A priori languages are constructed languages where the vocabulary is invented directly, rather than being derived from other existing languages (as with Esperanto or Ido). Philosophical languages are almost all a priori languages, but most a priori languages are not philosophical languages. For example, Quenya, Sindarin, and Klingon are all a priori but not philosophical languages: they are meant to seem like natural languages, even though they have no genetic relation to any natural languages.
History
Work on philosophical languages was pioneered by Francis Lodwick (A Common Writing, 1647; The Groundwork or Foundation laid (or So Intended) for the Framing of a New Perfect Language and a Universal Common Writing, 1652), Sir Thomas Urquhart (Logopandecteision, 1652), George Dalgarno (Ars signorum, 1661), and John Wilkins (An Essay towards a Real Character, and a Philosophical Language, 1668). Those were systems of hierarchical classification that were intended to result in both spoken and written expression. In 1855, English writer George Edmonds modified Wilkins' system, leaving its taxonomy intact, but changing the grammar, orthography and pronunciation of the language in an effort to make it easier to speak and to read.
Gottfried Leibniz created lingua generalis (or lingua universalis) in 1678, aiming to create a lexicon of characters upon which the user might perform calculations that would yield true propositions automatically; as a side effect he developed binary calculus.
These projects aimed not only to reduce or model grammar, but also to arrange all human knowledge into "characters" or hierarchies. This idea ultimately led to the Encyclopédie, in the Age of Enlightenment. Leibniz and the encyclopedists realized that it is impossible to organize human knowledge unequivocally as a tree, and so impossible to construct an a priori language based on such a classification of concepts. Under the entry Charactère, D'Alembert critically reviewed the projects of philosophical languages of the preceding century.
After the Encyclopédie, projects for a priori languages moved more and more to the fringe. Individual authors, typically unaware of the history of the idea, continued to propose taxonomic philosophical languages until the early 20th century (for example, Ro). More recent philosophical languages have usually moved away from taxonomic schemata, such as 21st century Ithkuil by John Quijada.