Interlingua and eligibility of international words

Words can be included in Interlingua in either of two ways: through regular derivation using roots and affixes or by establishing their eligibility as international words. The second of these methods is explained below.

The theory underlying Interlingua posits an international vocabulary, a large number of words and affixes that are present in a wide range of languages. Social forces, most notably the dynamism of science and technology, have spread this vocabulary to "all corners of the world". The goal of the International Auxiliary Language Association was to accept into Interlingua every widely international word in whatever languages it occurred. For practical reasons, however, IALA's researchers could not examine all the world's languages. Therefore, they conducted studies to identify a small group of languages that would deliver "the most generally international vocabulary possible", while still maintaining the unity of the language.

The languages selected are called control languages. The primary controls are English, French, Italian, Spanish, and Portuguese, with Spanish and Portuguese taken as one language. The secondary controls are German and Russian. According to the rule of three, a word is eligible for Interlingua if it occurs in at least three of the four primary control languages, with either or both of the secondary control languages acting as possible substitutes.

To provide generally international words, the control languages had to have a high degree of radiating power and a high degree of receptive power. In particular, they had to "radiate" a large number of words into other languages and to "absorb" a large number from other languages. Thus, IALA developed the concepts of productive and receptive spheres of language.

In classical antiquity, Latin and Greek were the languages of the dominant political and cultural forces. Afterward, these languages acted for two millennia as essential lingua francas in Western science and religion. As a result, the Western languages have imported many thousands of Greek and Latin words, either through ancestry or through transfer and loan. Many of these same words are found in non-Western languages, such as Arabic, Hindi, Swahili, and Japanese.

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