This English version was translated from the Chinese original with AI assistance. The Chinese text is authoritative.
“一” (yī, “one”) can have many meanings. One meaning is the number “1,” meaning one or first; it is the unit by which countable things are measured. In the Book of Changes it represents the yang line. In Chinese characters, “一” is a most basic component, called the horizontal stroke. Because this “一” comes from the Dao, it often expresses the meaning of principle, rule, law and so on. For example, the requirement in writing a horizontal stroke is that it be level and straight; the principle of being a person is to be upright; the principle of doing things is that a straight line is the shortest distance between two points, that is, be direct and concise. When used in Chinese characters, it also implies reasoning, principle and the like.
As a single character, it is pronounced “yī,” the same as “咦” (yí, an exclamation of surprise), only in a different tone. When the morning sun first rises, the horizon appears like a “一.” On seeing it, people’s first reaction is: “Yí, the sun has risen!” So it was quite natural to read this “一” with the sound of “咦,” only with the tone lowered.
What was the first Chinese character? This can be explained starting from the idiom 一画开天 (“one stroke opened heaven”). This idiom comes from the poem “Reading the Changes” by Lu You of the Song dynasty. According to legend, when Fuxi drew the Eight Trigrams, he began with the first line of the Qian trigram (☰); Qian is heaven, hence “one stroke opened heaven.” Laozi’s Daodejing has a saying: The Dao gives birth to one, one gives birth to two, two gives birth to three, and three gives birth to the myriad things. This saying tells how all things arise from the Dao. The Changes is one form of the Dao, so it likewise tells how “一” was born from the Dao. All things begin from one, so there should be no doubt that the first Chinese character is “一.” With this “一,” two and three can be born, and so on to all things, and naturally all Chinese characters can be born too. This also accords with the legend of Pangu splitting heaven and earth apart from chaos with one stroke of his axe.
The yang line “一” represents the world beginning to change out of chaos, carrying the meaning of change and of the rules and principles of change. Thus the character “一,” written the same way, became linked with “易” (yì, “change”), the character in Chinese that specifically expresses the rules of change, and so these two characters have the same pronunciation. From this follows a basic law or rule of the pronunciation of Chinese characters: the same sound means the same meaning, or the opposite meaning, or a related meaning.
