भयद्सग्निस्तपति भयात्तपति सूर्यः ||
भयादिन्द्रश्च वायुश्च मृतुर्धावति पञ्चम: ||३||
Kathopanishad Chapter 2 valli (shloka/verse) 3
From fear of bramhan fire burns, from fear(of bramhan) the sun shines, from fear Indra, Vayu, Death, and the fifth, run.
What I think 'The fifth' means :
The fifth element here might represent, Akasha (Ether/space) because agni(fire), vayu(air) are explicitly mentioned and from Indra -> varuna (water) is indirectly mentioned and bhoomi (earth) cannot run. So this is what I conclude on basis of omnipotence and importance of akasha element.
I think there is more to
the fifth in this verse, so I ask you(reader) to convince and prove(with literature) meaning of