Lemons are a definite species, but no-one knows all about them. In particular, a child may be barely able to tell lemons from other yellow fruit but nevertheless has a concept of lemon that may be adequate to find the lemons in the supermarket. The child can improve its concept of lemon by learning more. The fact that there isn't a continuum of fruits ranging from lemons to grapefruit is an important part of the fact that lemons form a natural kind. On the other hand, the legal concept of a person's taxable income is refined by defining more.
Taxable income is partly a natural kind.
A person's concept of his own taxable income is an approximation to the less approximate legal concept. He could learn more or it could be defined more as the legal concept is defined more. However, learning more about the legal concept eventually reaches a point where there is no further refinement on which people thinking independently will agree. There isn't a true notion of taxable income for economists to discover. My concept of my taxable income and even my tax accountant's concept of my taxable income has both aspects.