( ( The ( physical principle ) ) ( here ( ( ( is not ) ( ( ( ( one ( of density ) ) , ) but ) ( ( a combination ) ( of ( particle ( ( packing and ) inertia ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (DT The) (JJ physical) (NN principle)) (ADVP (RB here)) (VP (VBZ is) (RB not) (NP (NP (NP (CD one)) (PP (IN of) (NP (NN density)))) (, ,) (CC but) (NP (NP (DT a) (NN combination)) (PP (IN of) (NP (NN particle) (NN packing) (CC and) (NN inertia)))))) (. .)))
The physical principle here is not one of density, but a combination of particle packing and inertia.
(ROOT (S (NP (NP (NNP Density)) (PP (IN of) (NP (NN matter)))) (VP (VBZ reflects) (SBAR (WHADVP (WRB how) (RB closely)) (S (VP (VBD packed) (SBAR (S (NP (NP (DT the) (NNS particles)) (PP (IN of) (NP (NN matter)))) (VP (VBP are)))))))) (. .)))