Writing is not language.
How?Doesn't the sentence — that I underlined in red on p 280 — contradict this? The author wrote that if "language is manifested instead as graphic marks, then we can call those marks writing".
We can define writing as the use of graphic marks to represent specific linguistic utterances. The purpose of a definition is to distinguish a term from other things. To understand what writing is, it is helpful to investigate some similar things which are not writing according to our definition.
Writing is not language.
Language is a complex system residing in our brain which allows us to produce and interpret utterances. Writing involves making an utterance visible. Our cultural tradition does not make this distinction clearly. We sometimes hear statements such as Hebrew has no vowels; this statement is roughly true for the Hebrew writing system, but it is definitely not true for the Hebrew language. Readers should constantly check that they are not confusing language and writing.
Henry Rogers, Writing Systems (2004), p 2.
Op. cit., p 280.