Military expansion. The Rashidun Caliphate expanded gradually; within the span of 24 years of conquest, a vast territory was conquered comprising Mesopotamia, the Levant, parts of Anatolia, and most of the Sasanian Empire. Unlike the Sasanian Persians, the Byzantines, after losing Syria, retreated back to Anatolia.
1 And the nature of this reality again can neither be consistently represented as a fixed and hard substance nor as an unalterable something, but only as a fixed order of recurrence of continually changing events or impressions.
(adjective) 1) continually occurring; persistent. 2) regularly recurring. 3) unchanging in nature, value, or extent; invariable. 4) steadfast in purpose, loyalty, or affection; faithful. A constant in this case refers to the loyalty of an object, thing or person in our life.