non secundum necessitatem naturae; Ergo non est necessarium secundum se.

The fallacy of inferring from the fact that F is accidental (or not necessary) to a thing’s nature, that therefore F is accidental (or not necessary) to the thing.

Example 1:

Being female is accidental with respect to being human; therefore Jane is not necessarily female. 

Example 2:

Being born to Carol and Richard is not an essential feature of my human nature; therefore I could have had different parents.

Example 3:

Being composed of leather is not essential to being a couch; therefore this leather couch could have been composed of cloth.

Comment: Though I agree that each of these examples commits the non secundum necessitatem naturae; Ergo non est necessarium secundum se fallacy, I still affirm the conclusions of examples 1 and 2.