Sentence:  Whitehead is here questioning David Hume 's <WSD>understanding</WSD> of the nature 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 cognitive condition of someone who understands