Just as light and shadow are opposites, Nibbana is the opposite of the formed:http://www.beyondthenet.net/dhamma/nibbanaReal.htm
...and the opposite of the differentiated:
"papañca: (Sanskrit prapañca): In doctrinal usage, it signifies the expansion, differentiation, 'diffuseness' or 'manifoldness' of the world; and it may also refer to the 'phenomenal world' in general, and to the mental attitude of 'worldliness'. In A. IV, 173, it is said: "As far as the field of sixfold sense-impression extends, so far reaches the world of diffuseness (or the phenomenal world; papañcassa gati); as far as the world of diffuseness extends, so far extends the field of sixfold sense-impression. Through the complete fading away and cessation of the field of sixfold sense-impression, there comes about the cessation and the coming-to-rest of the world of diffuseness (papañca-nirodho papañca-vupasamo)."
The opposite term nippapañca is a name for Nibbána (S. LIII), in the sense of 'freedom from samsaric diffuseness'. - Dhp. 254: "Mankind delights in the diffuseness of the world, the Perfect Ones are free from such diffuseness" (papañcábhiratá pajá, nippapañca tathágatá). - The 8th of the 'thoughts of a great man' (mahá-purisa-vitakka; A. VIII, 30) has: "This Dhamma is for one who delights in non-diffuseness (the unworldly, Nibbána); it is not for him who delights in worldliness (papañca)." - For the psychological sense of 'differentiation', see M. 18 (Madhupindika Sutta): "Whatever man conceives (vitakketi) that he differentiates (papañceti); and what he differentiates, by reason thereof ideas and considerations of differentiation (papañca-saññá-sankhá) arise in him."---"Buddhist Dictionary", Nyanatiloka.