From a previous conversation on the topic (Im sure to muck this up, but anyhoo), he said the best way to conceptualize fatigue is to see it as the collective state of physiological, perceptual and performance indices.
And what really got my thinking was how he emphasized "Higher Control" functions frequently breakdown when one is in a stressful setting and fatigued, and how it especially leads to impairment to "assess and respond to Perceived Demands"
I believe this is the area he feels is most relevant to our profession.
Edit to add: Whenever we discuss the actions of a flight crew and say "what were they thinking?", I too wonder how often it all comes down to fatigue, plain and simple.