1. Intellectual Background

One cannot ascribe a single outlook to all Esperantists or all Interlinguists; however, the contrasting views of L. L. Zamenhof and Alexander Gode remain influential among Esperantists and Interlinguists, respectively. Zamenhof, the inventor of Esperanto, was motivated by several strands of nineteenth-century idealism, ranging from Comtean positivism to utopian internationalism. Esperanto, in his view, was a theoretically neutral instrument for communication, which could serve as a vehicle for idealistic values, initially Zamenhof's philosophy of homaranismo, later the interna ideo (internal idea) of achieving "fraternity and justice among all people" (Zamenhof) through the adoption of Esperanto. Among later Esperantists, this philosophy has tended to reinforce a set of propositions about the language:

• Esperanto's European character is purely accidental; however, some features of Esperanto (and of some western languages) can be found in non-Western ones.
• Esperanto is, ideally, the universal second language, replacing all other languages in inter-ethnic communication; pro-Esperanto arguments tend to assume a future situation of widespread Esperanto use in many situations where English is currently dominant.
• A tension exists between finvenkismo, predicting a 'final victory' (fina venko) of Esperanto, and Raŭmismo, which considers the 'final victory' too remote a goal.
• Esperanto is a vehicle for a specific internationalist and humanitarian ideology.
• Cultivation of an internal Esperanto culture is an important value for many Esperantists.

By the mid-twentieth century, when Gode led the development of Interlingua, the ideals underlying Esperanto had come to seem naive. Influenced by Herder, Gode propounded a Romantic, anti-positivist view of language: languages are an aspect of the culture of a people, not an instrument to achieve a goal; an ideology cannot be attached to a language, except artificially. This implied, in his view, that a world language on the Esperanto model was either impossible or, worse, achievable only through totalitarian coercion. He was of the opinion that, unless imposed by force, a universal global language would presuppose a universal global culture, which does not currently exist and is not necessarily desirable.

On the other hand, Gode saw another sort of international language—non-universal and non-culturally neutral—as being entirely possible.

2. Learnability Versus Comprehensibility

Esperanto and Interlingua are fundamentally different in their purposes. Whereas Esperanto is meant to be an international second language able to be fluently learned by speakers of any language, Interlingua is directed more toward European languages, especially its control languages. Though Esperanto may be more neutral as well as easier to master, Esperantists can usually communicate only with other Esperantists, though the language is at least somewhat incomprehensible to many who have not studied it; Interlingua, however, is intended to be understood to some extent by a speaker of any Romance language, because many words in Interlingua resemble their originals in Latin, the precursor to all Romance languages.

3. Vocabulary

The vocabulary of both languages is taken largely from Romance, Germanic and Slavic languages; most of these words are derived from Latin. Depending on their international form, Germanic and Slavic words in Interlingua may be Latinized; for example, English blockade, German Blockade, Russian блокада → Interlingua blocada. By comparison, all words in Esperanto take on a characteristic Esperanto form. In this case, the Interlingua blocada and the Esperanto blokado are nearly identical and equally neutral.

Though both Esperanto and Interlingua borrow primarily from European languages, they also borrow words from other languages which have become widespread. Two different philosophies have led to two different approaches. Interlingua highly regards etymological fidelity, thus it usually adopts the word that is the nearest common ancestor of the respective words in at least three source language units (considering Spanish and Portuguese together as one unit). Esperanto highly regards regularity, thus it disregards the form of the word in European languages to make it match Esperanto's morphology and phonemic orthography. For example, Interlingua has geisha (from Japanese 芸者), sheik (from Arabic شيخ), and kayak (from Inuit ᖃᔭᖅ); in Esperanto, these words are written gejŝo, ŝejko, and kajako.

In Esperanto, to form a new word, it is generally preferred to compound two or more existing roots than to borrow a word from another language. This is recommended in order to keep the number of "primitive" roots low and thus to maintain its learnability. Interlingua does not have that as a design aim, thus most of its compound and "primitive" (non-compound) words also exist in its source languages.

4. Grammar and Word Formation

Both languages have a highly regular grammar without difficult conjugations or declensions. However, Interlingua lacks adjective agreement and case endings, which make its noun and adjective morphology simpler.

Interlingua draws its roots from certain "control languages": French, Italian, Spanish, Portuguese, English, German and Russian. It uses these languages as a means to select the words most used in these major European languages. Esperanto draws from largely the same languages, but uses agglutination more extensively. Rather than using an existing word commonly used among the major European languages, Esperanto forms its own words using its own roots. For example, the Esperanto word for "hospital" is mal·san·ul·ej·o, which breaks down into five roots: mal (opposite), san (health), ul (person), ej (place), o (noun). Interlingua tends to use words derived from natural languages instead of extensive agglutination.

The following table illustrates the difference between Esperanto and Interlingua with regards to word formation:

To the reader who speaks English or a Romance language, the words in the Interlingua column are more likely to appear recognizable. However, speakers of languages that do not have words related to those in the Interlingua column must learn the Interlingua words one at a time; the Esperanto words, however, would be simple to use because of its system of word derivation. This point underlines the fundamental differences between Esperanto and Interlingua: the latter was designed to be easily understood by speakers of European languages, whereas the former was designed for people to learn to speak more easily. (Interlingua does have, however, a more regular system of word derivation than many natural languages.)

Often, the European words on which Interlingua is based gain extensive currency in non-Western languages. Hospital, for example, appears in Celtic languages such as Irish, Scottish Gaelic, Manx, and Breton; Germanic languages such as Dutch, Danish and Afrikaans; Slavic languages such as Ukrainian and Polish; Indo-Aryan languages such as Hindi, Urdu and Bengali; Altaic languages such as Mongolian, Turkmen and Azerbaijani; Austronesian languages such as Indonesian, Ilocano, Tagalog, Tetum and Chamorro; African languages such as Swahili, Kongo and Setswana; and creoles and linguistically isolated languages such as Papiamento, Albanian, Mapunzugun and Basque.^[1][2] In many other languages, however, the word hospital is not found, including Finnish, Arabic, Hebrew, Vietnamese and Hungarian.

Both languages attempt to be as precise as possible; that is, each strives to reflect differences in meaning using different words. The Esperanto compound mal-san-ul-ej-o, literally "un-healthy-person-place-noun", implies a place for people who are unhealthy. The word means "hospital", but the compound could be construed as any place where an unhealthy person is. Interlingua's non-compound word, though possibly less neutral, thus avoids any misunderstanding. (Depending on the speaker and audience, Esperanto could also use a different word for "hospital", such as hospitalo, kliniko, lazareto, preventorio or sanatorio.)

Interlingua and Esperanto have minor differences regarding precisely how agglutinations occur. For example, Interlingua adds tense endings to the indicative form of a verb (dona → donar), while Esperanto adds them to the stem (don- → doni).

5. Orthography

The orthography of Esperanto is inspired by the Latin alphabets of Slavic languages, and is almost completely phonemic (one sound, one letter). Interlingua, by contrast, uses an orthography established by its Romance, Germanic, and Slavic source languages. Thus, the orthography of Interlingua is much more broad-based than that of Esperanto. The procedure used sometimes favored English and the Romance languages, however, resulting in a little less phonemicity and a little more familiarity to speakers of those languages.

For example, the Esperanto kontakto and the Interlingua contacto mean the same thing and are pronounced the same, but are written differently, because the orthography of Esperanto is simpler: one sound, one letter. Interlingua occasionally departs from this rule, chiefly because the letters "c" and "g" have hard and soft sounds. Such details make Interlingua more difficult to learn and speak for people who are unfamiliar with English and the Romance languages, but at the same time easier to read and understand for speakers of Romance or Romance-influenced languages: the letters show the history of the Romance influence. The difference, however, is minimal.

The use of diacritics in Esperanto historically made it more difficult to type on standard typewriters and older computers; since support for Unicode has become widespread, this is less of an issue, but still may require some setup tweaks for users who don't otherwise use a language with diacritics. Alternately, a widely accepted standard is to signify the letters with diacritics by simply typing the letter "x," which is not part of its alphabet, after the affected letter. On the other hand, Interlingua uses only the basic Latin alphabet, with no diacritics.

6. Expressiveness

Supporters of Interlingua note that their language not only conserves the natural aspect of Western languages, but also their rich, subtle treasury of meanings. Interlingua flows regularly from its Romance, Germanic, and Slavic source languages, and thus it possesses their expressiveness.

Esperanto supporters contend that, by its liberal use of affixes and its flexible word-order, is equally as expressive as Interlingua or indeed any natural language, but is more internationally neutral. While acknowledging that Esperanto is a product of rational construction, not historical evolution, they argue that, after the prolonged usage of more than 100 years, it too has become a living human language.

7. Number of Speakers

Although no census has ever been undertaken, Esperanto speakers frequently place their numbers at somewhere between 100,000 and 3 million speakers. The number of Interlingua speakers is generally estimated between a few hundred and 1,500. Esperanto is the only constructed language with native speakers, numbering 200-2000 according to Ethnologue.^[3]

8. Sample Text: the Lord's Prayer