The book illustrates the negative temporal proposition by mentioning the moon at quadrature and denying eclipse under that specified condition.
«بالضرورة لا شيء من القمر منخسف وقت التربيع لا دائما»
Necessarily the moon is not eclipsed at quadrature, but this denial is not permanent.
The initial component is negative: it denies eclipse at the designated time. The final “not permanently” says that absence of eclipse cannot be generalized across the moon's entire duration.
The distinction is between “not eclipsed at this specified time” and “never eclipsed.”