Sentence:  Whitehead is here questioning David Hume 's understanding 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 , <WSD>not</WSD> directly perceived . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words