The quality or state of being connatural; the condition of sharing the same nature or being inherently related.
From 'connatural' plus the suffix '-ity', which forms abstract nouns describing qualities or conditions. This creates the noun form to discuss the abstract concept of natural connection.
This is an ultra-specialized philosophical term—you'll find it in medieval and early modern theology and philosophy but almost nowhere else, making it a perfect example of how academic disciplines develop their own hyper-precise vocabulary.
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