Little doubt, English is the single dominant global and scientific language in the world today. Mainstream opinions attribute its rise to political, economic, technological and cultural causes, through the influences of the British empire and the USA.
However, from the standpoint of our previous papers, our argument is straightforward: legibility  is the cause. The evolutionary account was addressed in my paper “visual evolution of written language towards Latin alphabet – a hypothesis”. Writing’s effects on the world were discussed in “effects of written language on the non-textual world” . We now explicitly discuss this topic is to correct the misconceptions and affirm our argument which can be used to explain the history and present societies, and to predict the future.
1. “Being fit for reading” is the cause
We should treat writing as a tool, just like other tools, such as a pen. One pen is better than another not because the person who uses it has better writing skills, although he/she would likely to choose a better pen. Likewise, a writing system is better not because its people are more powerful. Writing is useful because it is uniformly arranged. The legibility of the arrangement certainly has an effect on its usefulness. Different writing systems have varied levels of legibility. When other conditions are the same, people prefer reading/writing legible  texts to illegible ones. Legibility (fitness for reading) is the rudimentary criterion that people choose a writing system from existing ones or develop new systems.
The letters, words and sentences are refined visually by millions of people. The visual refinement of written English is unmatched by other systems. That enables the vast publications and spread of the English writing system, maximizing the established visual, textual information. Legible systems attract more people to contribute to its expansion. Although Mandarin and Spanish have more native speakers than English, their spreads are limited due to most advanced knowledge, which is the reason for language learning, are written in English. With fitness for reading, English writings are relatively fixed and reasonable. Its speakers are firm about that, influencing the people of illegible systems. When two or more writing systems meet, each absorbs others’ vocabulary, grammar, etc. to grow themselves. The English turned out to be stronger in this aspect.
Of course, legibility is not the only reason for a writing system to spread. Official status, the existing number of users, available publications, etc. all influence the spread’s momentum. But legibility is the ultimate cause, either due to voluntary adoption of legible systems or legible systems growing strong to overpower illegible ones. Illegible systems will evolve in legible direction or be replaced by legible systems, which might be initially smaller.
2. The misconceptions
2.1. Nonlinguistic influences, or non-textual causes
This view attributes the causes to external factors instead of language itself, for instances, political, economic, technological, and cultural causes. These factors empower the people, whose languages will consequently dominate. People from less advanced societies would voluntarily or be forced to learn the languages of more advanced societies. To an extreme, illiterate people will eliminate all texts if they are powerful enough. Generally, this view would consider that advanced societies need legible systems, but not necessarily.
We argue that legibility factor is ultimate and deterministic. We consider texts as the center of the human-created world. Legible texts establish advanced societies, which can be implied from The Paper. That is to say, social progress is built upon a specific writing system, which spread with its societies and people. Political, economic and other situations might fluctuate, for months or even years. An illegible writing system might prevail temporarily. On the long run, legible systems would build advanced societies and overpower illegible systems.
2.2. Spoken form is the primary form of language
This view draws people’s attention away from the written form. Many people concentrate on the spoken forms when comparing two languages, marginalizing writing or not even starting to think about the effects of writing’s visual aspect. They certainly cannot get into the heart of the problem, not to mention finding solutions. Sound representing is a major disguise of the legibility of scripts. The truth is: the spread and dominance of oral English is due to the success of English texts. The course of English globalization is the governance of speech by the written text.
2.3. Simple spelling contributes to English spread
Many people know that English spelling is simple, thus easy to learn and use, contributing to English spread. This opinion starts to look at the written form. However, simple spelling is usually considered a trivial factor. Moreover, their opinion assumes spoken English is primary, which the spelling represents.
In sum, the root causes of the wrong understanding are not realizing written form as the primary form of language, and written language as the central cause of social progress.
3. Latin alphabet-based writing systems, English and other systems
Like English, the historical dominances of Latin, Spanish, German and French were all due to the Latin alphabet, although their spoken forms set them apart as different languages. Latin alphabet’s ultimate contribution is neglected. But these systems are not as refined in basic-unit level as English. The use of diacritics, ligatures and additional letters can make the scripts worse for reading. Complex spelling-sound correspondences and irregularities of spelling also provide English the flexibility to achieve legibility and facilitate vocabulary growth.
It is important to compare Latin alphabet-based systems with other systems, because the distinctions between Latin alphabet-based systems are relatively small, obscuring the significance of the legibility factor. Compared with other scripts, Latin script’s letters, particular lowercase letters, are better shaped with clarity, simplicity and inter-letter differentiation. Latin letters also make up syllables, words in legible manners. The prevalence of romanization indicates the high legibility of Latin alphabet, often under the disguise of spelling to represent sounds of the languages of non-Latin scripts.
4. The future
It is likely that English will continue to grow and evolve based on current alphabet, vocabulary and grammar. It will speed up its spread during globalization and the revolution of the media. It will be enriched by regional lexicon, creole and pidgin, dialects and varieties. More writing systems and languages might be dying out, regretfully. But the destiny of written language is to regulate, organize, and maximize the capability of the mind, to support larger societies, and to encode the world. Diversity is not the goal of written language. Many writing systems will be preserved or even grow, at least in the near future. With diversity preserved, a dominant writing system will be the foundation of mankind’s common societies and common minds, governing other systems.
 Herein, “legibility” and “fitness for reading” have the same meaning.
 Referred to as The Paper hereafter.
 Herein, “legible” and “illegible” are relative terms.