Full Idea
In the light of problems such as the definition of knowledge, many philosophers now take seriously the possibility that our concepts lack definitional structure.
Gist of Idea
It may be that our concepts (such as 'knowledge') have no definitional structure
Source
E Margolis/S Laurence (Concepts [2009], 2.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.8
A Reaction
This challenges the classical view, that there are precise conditions for each concept. That view would obviously be in difficulties with atomic concepts, so our account of those might be applied all the way up.