Sentence:  If we are another <WSD>footnote</WSD> to Plato, Plato was himself already a <WSD>footnote</WSD> to still earlier <WSD>footnotes</WSD>, in an endless chain of <WSD>footnotes</WSD> to <WSD>footnotes</WSD> 
Meaning of the word marked with <WSD> tags:  An event of lesser importance than some larger event to which it is related.