Full Idea
According to critics, the thorniest problem for essentialism is the question of our knowledge of essence. It is usually at this point that terms of abuse such as 'dark', 'mysterious', and 'occult' are wheeled out.
Gist of Idea
Critics say that essences are too mysterious to be known
Source
Scott Shalkowski (Essence and Being [2008], 'Ess and Know')
Book Reference
'Being: Developments in Contemporary Metaphysics', ed/tr. Le Poidevin,R [CUP 2008], p.59
A Reaction
I'm inclined to think that the existence of essences can be fairly conclusively inferred, but that attributing a precise identity to them is the biggest challenge.