«بالضرورة لا شيء من الكاتب بساكن الأصابع ما دام كاتبا لا دائما»
Necessarily, no writer is one whose fingers are still while writing, but not permanently.
The first component is negative: stillness is denied under the writing description. “Not permanently” then denies that the same negative relation holds over the subject's whole existence.
The two negative expressions have different targets:
“No writer ...” — denial of the predicate.
“Not permanently” — denial that this judgment persists throughout the subject's existence.