Full Idea
We can't define "consciousness" by necessary and sufficient conditions, or by the Aristotelian method of genus and differentia.
Clarification
Aristotle places a thing in a class, then identifies what distinguishes it from other members of the class
Gist of Idea
Consciousness seems indefinable by conditions or categories
Source
John Searle (The Rediscovery of the Mind [1992], Ch. 4.I)
Book Reference
Searle,John R.: 'The Rediscovery of the Mind' [MIT 1999], p.83
A Reaction
We may not be able to 'define' it, but we can 'characterise' it. The third approach to definition is a catalogue of essential properties, which might tail off rather vaguely.