No preferred directions in space.
Definition of Isotropy
Qualitatively, isotropy refers to the absence of preferred directions in space.
'Primordial Cosmology', Montani Battisti Benini Imponente, World Scientific (2011) page 97.
So there are no preferred directions in space however 1400 years ago this was portrayed in the Quran:
[Quran 24.35] Allah is the Light of the heavens and the earth. The allegory of His light is that of a pillar on which is a lamp. The lamp is within a glass. The glass is like a brilliant planet, fueled by a blessed tree, an olive tree, neither eastern nor western. Its oil would almost illuminate, even if no fire has touched it. Light upon Light. Allah guides to His light whomever He wills. Allah thus cites the parables for the people. Allah is cognizant of everything.
"Neither eastern nor western" means no preferred directions in space.
How could an illiterate man who lived 1400 years ago have known that there are no preferred directions in space?
For olive oil emitting light without burning see: Fluorescence