If we see that 3, 5, 13, and 19 are positive instances of a category and 4, 14, and 28 are negative instances, then a simple boundary which separates these instances is "All odd numbers\." A simple and conservative boundary is "All odd numbers between 3 and 19" or "All primes between 3 and 19"\. \(A non\-simple boundary is "Only 3, 5, 13, and 19 are members of the category\."\)
Is this really more complex than "All primes between 3 and 19"? I think you need more numbers before you can import the definition of prime and have that be simple.