We shall now proceed to the question of a vocabulary. In choosing the majority of his stems, Dr. Zamenhof had already followed the principle of maximum internationality, but the authors of Neutral Idiom were the first to carry out this principle scientifically for the whole language. Their procedure was, however, somewhat superficial, since in each particular case they calculated the number of languages to which a given word was common. One must not count the languages (and Latin especially must not be counted along with the living languages), but the people who use them, for languages are not organisms which possess an individual existence independent of those who speak them. The proper rule, therefore, for determining the internationality of a word or stem is to count the number of people who understand it through their mother tongue. This definition of the principle of maximum internationality is simply a necessary consequence of the fundamental principle of the greatest facility for the greatest number. It is natural that each person would prefer the use of the greatest number of words which are familiar to him, and so, to be impartial, we must attach the same value to the individual preferences of the 120,000,000 who speak English as to those of the 75,000,000 Germans, the 70,000,000 Russians, or the 50,000,000 French or Spanish, etc. Even the languages spoken by the smaller nations must be taken into account in proportion to their numbers.
The choice of the words for our neutral language is, therefore, a pure question of arithmetic. Statistics of the number of people who speak the different languages will not, however, furnish us with a complete solution of the problem. In the first place, there are to be found in the dictionaries technical words and special terms which are only known to a minority of each nation. In the second place, there occur cases where a word, though it does not belong to a language, is, nevertheless, known through one or more derivatives. For example, 100 is in English hundred, in German hundert, in Danish hundrede, and yet the root cent (zent) has been long familiar to the world through the terms per cent. (G. prozent), centesimal, centimetre, centennial, century, centenary, G. zentner, Danish centner. In the third place, even when "the same word" belongs to several languages, it very often possesses different forms, due mostly to a different phonetic development, with the result that the choice of a proper form is very often a delicate matter. The sounds of the word "change," which the English and French write in the same way, are very different; but as we can employ neither the nasal vowel of the French nor the diphthong (ei) of the most usual English pronunciation, chanj would appear to be the most convenient form for all. In very many cases it is possible to find a common denominator for the different forms. Had not in English and German the external form of many etymologically closely related words diverged so much that it is impossible to find a middle form (for example, water, wasser; tooth, zahn; speak, sprechen; soap, seife; week, woche), the Germanic element would have been the dominating one on account of the great number of those speaking these two related languages. Such being the case, the Romance element in English usually decides the matter in the majority of instances, since it coincides with the French, Spanish, and Italian, or at least with one of these languages, the result being that our language necessarily possesses a Romance form in a much higher degree than one might have thought. Another very important circumstance (which I have hinted at previously) acts in the same direction, the circumstance, namely, that numerous Latin derivatives have passed over into the Germanic languages even when the stem does not occur there. For example, German possesses the words absentieren, abstinenz, artist, dentist, dental, moral, populär, which greatly facilitate for a German the understanding of the words absenta, abstenar, arto, dento, moro, populo, although he does not possess them in his own language (with the exception of pöbel = populacho).
Sometimes there exists a very troublesome rivalry between two words. In order to render the substantive "arm" (limb) the proper word would seem to be the German, English, and Scandinavian "arm," until one makes the discovery that the same root "arm" in the sense of "weapon" is still more international (E., F., I., S., supported by armée G., E., F., R., armata I., armada S., armieren G., etc.), which compels us for "arm" (limb) to have recourse to a Romance form. In other cases a more or less arbitrary change of one of the series of words appears to be the only means of avoiding confusing homonyms (namely, for door pordo instead of porto, on account of port = carry), but this procedure must be employed with great caution. Before everything else it is necessary to avoid all disguising of words, which makes them unrecognisable, aptly described by M. Blondel as a masquerade. This was set up as a general principle in Volapük, and Esperanto is by no means free from it.
As an example of the conflicts which occur now and then may be quoted the expressions for the idea of "soul." "Soul" is the word which would be immediately understood by the greatest number of people, but we cannot employ the English diphthong ōū, as we must be very sparing in the use of diphthongs, since they cause very great difficulties in pronunciation. We cannot take over the word in the form sol, because we require this for the word "alone" (I. S. solo, internationally used in music, E. sole, F. seul). G. seele, supported by the Scandinavian själ, is not familiar to a sufficient number of people, and, besides, we require the word sel for "saddle" (F. I. S.). The French word âme will not do either, because it is not sufficiently well known outside France, and, besides, there is a difficulty here too, for am- is absolutely required for the idea of "love" on account of F. I. S. and many derivatives in E., not to mention the god Amor. The use of the Latin anim-, which is the basis of the Romance forms, is impossible, since we cannot do without the adjectival termination -al, and animal would then mean partly "relating to the soul," partly "animal," which cannot be permitted in an international language. We must resort to the device of changing anim- a little, whereby we get anmo. This example will show how complicated the task frequently is of finding an international word which will give rise to no confusion or misunderstanding.
The degree of internationality of the language of the Delegation will be evident from the statistics of Couturat; he counted the roots of the first dictionaries (5,379 in all) and found that of these the following numbers occur in the national languages:—
| French | 4,880, | i.e. | 91 | per 100 | |
| Italian | 4,454 | " | 83 | " | " |
| Spanish | 4,237 | " | 79 | " | " |
| English | 4,219 | " | 79 | " | " |
| German | 3,302 | " | 61 | " | " |
| Russian | 2,821 | " | 52 | " | " |
For all these languages the above numbers are relatively higher than in the case of Esperanto.
One of the most effective means of simplifying the vocabulary of a language is a carefully worked-out system of word formation, which enables everyone, by means of a series of regular prefixes and suffixes, to form with the greatest ease a large number of new words, which are immediately intelligible to all who know the rules.
When one has judiciously chosen the roots which occur under different forms in the various natural languages and also selected the derivative terminations with all possible care, it is astonishing to observe how great a number of words derived with perfect regularity agree with the forms occurring in living languages.
With regard to grammar, the fundamental condition to be required of every system claiming to be an international language is that of perfect regularity. Every exception to the rules only serves to produce complications and to render the employment of the language difficult and uncertain. If one knows the conjugation of one verb, one must know the conjugation of all verbs, and so on.