This is very similar to a problem in linguistics (and in some ways &quot;is&quot; a linguistics problem).<p>You can define what an acorn is, and you can define a tree, but when does an acorn become a tree?  Same with baldness.  Everyone can identify a bald person and likewise someone who isn&#x27;t bald, but when do you go from not bald to bald?  This incidentally is part of the fundamental debate about abortion (when does a fetus become a child?).<p>And it is the same here -- we can define a human and &quot;not human&quot; but we&#x27;re having trouble defining exactly when it goes from one to the next.