I'd argue that this is FAR more true of the soft sciences - of which biology is perhaps the "hardest".

The definition of steel is pretty hard-edged. A polymer of C, H, and O isn't steel.

The definition of a quasar is similar. There are "maybe quasars", but that's from lack of data, not lack of definition clarity.

A circle is pretty exactingly defined. Mathematicians aren't fuzzy on that.