Sentence:  Whitehead is here questioning David Hume 's understanding of the <WSD>nature</WSD> of experience ; he is questioning , also , every epistemology which stems from Hume 's presupposition that experience is merely sense data in abstraction from causal efficacy , and that causal efficacy is something intellectually imputed to the world , not directly perceived . 
Meaning of the word marked with <WSD> tags:  the essential qualities or characteristics by which something is recognized