-ность

Probably originated from the suffix -ость (-ostʹ) added to words ending in -ный (-nyj), and was then extracted by rebracketing as a productive suffix. The same thing happened with e.g. -тельный (-telʹnyj) and English -ness.

-ность • (-nostʹ)

1. Suffix forming nouns denoting the quality of the action designated by the verbs from which the nouns are formed.

   погреши́ть (pogrešítʹ, “to err”) → погре́шность (pogréšnostʹ, “mistake, error, fault”)

   принадлежа́ть (prinadležátʹ, “to belong”) → принадле́жность (prinadléžnostʹ, “belonging, accessory”)

   провини́ться (provinítʹsja, “to commit an offence”) → прови́нность (provínnostʹ, “misconduct, offence”)

2. Suffix forming abstract nouns from adjectives. A less productive alternative to -ость (-ostʹ), except after щ, where it is the regular suffix.

   гото́вый (gotóvyj, “ready”) → гото́вность (gotóvnostʹ, “readiness”) (no such word *гото́вный exists)

   о́бщий (óbščij, “general, common”) → о́бщность (óbščnostʹ, “generality, community”) (no such word *о́бщный exists)

3. Suffix forming nouns with the meaning of a group of things characterized by the quality designated by the adjectives from which the nouns are formed.

   живо́й (živój, “living”) → жи́вность (žívnostʹ, “small creatures collectively”)

4. Suffix forming nouns of various sorts from miscellaneous parts of speech.

   пове́рх (povérx, “over”) → пове́рхность (povérxnostʹ, “surface”)

• Not all words in -ность (-nostʹ) contain this suffix. Most of them are formed from -ость (-ostʹ) added to adjectives in -ный (-nyj), e.g. вре́дность (vrédnostʹ, “harmfulness”) from вре́дный (vrédnyj, “harmful”).
• As noted above, -ность (-nostʹ) is the regular alternative to -ость (-ostʹ) after adjectives whose stem ends in щ, e.g. о́бщий (óbščij), су́щий (súščij), бу́дущий (búduščij).

• T. F. Yefremova (2006), The New Dictionary of the Russian Language