Say I am trying to define "apple", I define it in terms of a specific way by which atoms, molecules, and ions are organized. Then I'd have to define those atoms, molecules and ions, and I do so in terms of elementary physical particles. But then those elementary physical particles, and the way they behave, remain "unexplained", i.e. I accept them as they are without question.
The result of this chain of definition is that, either I end up with an endless chain, or I stop somewhere where I just can't go any further, and I'd have to live with the fact that "it is what it is".
What is the name of this problem? What are the related terms I should look up?
Thanks a lot for any pointers!