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  • AlGhazalisIhya-Book_of_knowledge, page : 35

obligatory upon every Muslim”, is knowledge with the definite article, namely, the
knowledge of only those works, which are well known to be obligatory on every
Muslim, the line of this reasoning would become apparent as would also the time
in, which these obligations should be discharged. Allah, however, knows best.

On Knowledge whose acquisition is deemed Fard kifayah (Collective Duty)

It should be known that a necessary duty is not distinguished from other duties,
except when the different sciences are enumerated. These are divided in relation
to the kind of duties we are now considering, into the science of jurisprudence
and secular (ghayr shariyah) sciences. By the science of jurisprudence I mean
those that have been acquired from the prophets and are not arrived at either by
reason, like arithmetic, or by experimentation, like medicine or by hearing, like
language.

Secular sciences are divided into praiseworthy (mahmud), blameworthy
(madhmum), and permissible (mubah). Praiseworthy sciences are those on
whose knowledge the activities of this life depend such as medicine arithmetic.