What does "the grit of something" mean?

Question

I read this sentence:

those problem are the creative grit of a theory

What does "the creative grit" mean?

Answer 1

A sentence similar to the example in the question appears on page 48 of the book On Secularization: Towards a Revised General Theory, by David Martín. In this case “creative grit” apparently uses sense 1 of grit, “hard small materials, such as dirt, ground stone, debris from sandblasting or other such grinding” or perhaps sense 2, “Inedible particles in food”. Either way, the author's intention appears to be to figuratively suggest that a theory will grow, due to problems the theory addresses, in that same way that in oysters pearls start growing around small bits of grit.