The story of Babel in the Old Testament reflects the popular feeling that confusion of tongues is a hindrance and a curse. Similarly in the New Testament the Pentecostal gift of tongues is a direct gift of God. But apparently it was not till about 300 years ago that philosophers began to think seriously about a world-language.
The earliest attempts were based upon the mediaeval idea that man might attain to a perfect knowledge of the universe. The whole sum of things might, it was thought, be brought by division and subdivision within an orderly scheme of classification. To
any conceivable idea or thing capable of being represented by human speech might therefore be attached a corresponding word, like a label, on a perfectly regular and logical system. Words would thus be self-explanatory to any person who had grasped the system, and would serve as an index or key to the things they represented. Language thus became a branch of philosophy as the men of the time conceived it, or at all events a useful handmaid. Thus arose the idea of a "philosophical language."
A very simple illustration will serve to show what is meant. Go into a big library and look up any work in the catalogue. You will find a reference number—say, 04582.g. 35,c. If you learnt the system of classification of that library, the reference number would explain to you where to find that particular book out of any number of millions. The fact of the number beginning with a "0" would at once place the book in a certain main division, and so on with the other numbers, till "g" in that series gave you a fairly small subdivision. Within that, "35" gives you the number of the case, and "c" the shelf within the case. The book is soon run to earth.
Just so a word in a philosophical language. Suppose the word is brabo. The final o shows it to be a noun. The monosyllabic root shows it to be concrete. The initial b shows it to be in the animal category. The subsequent letters give subdivisions of the animal kingdom, till the word is narrowed down by its form to membership of one small class of animals. The other members of the class will be denoted by an ordered sequence of words in which only the letter denoting the individual is changed. Thus, if brabo means "dog," braco may be "cat," and so on: brado, brafo, brago... etc., according to the classification set up.
Words, then, are reduced to mere formulae; and grammar, inflections, etc., are similarly laid out on purely logical, systematic lines, without taking any account of existing languages and their structure. To languages of this type the historians of the universal language have given the name of a priori languages.
Directly opposed to these is the other group of artificial languages, called a posteriori. These are wholly based on the principle of borrowing from existing language: their artificiality consists in choice of words and in regularization and simplification of vocabulary and grammar. They avoid, as far as possible, any elements of arbitrary invention, and confine themselves to adapting and making easier what usage has already sanctioned.
Between the two main types come the mixed languages, partaking of the nature of each.
The following list is taken from the Histoire de la langue universelle, by MM. Couturat and Leau: