Full Idea
Avicenna's 'indifference of essence' says the essence of certain things can become universal or singular, according to whether it is entertained by the mind (as a universal) or concretely exemplified as a singular thing. One essence can exist in two ways.
Gist of Idea
An essence can either be universal (in the mind) or singular (in concrete particulars)
Source
report of Avicenna (Abu Ibn Sina) (Commentary on the Metaphysics [1022]) by Claude Panaccio - Medieval Problem of Universals 'Sources'
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.50
A Reaction
This would appear to be a form of nominalism, since in the concrete external world we only have particulars, and it is our mode of thinking (by abstraction?) that generates the universal aspect. I think this is probably right.