"Imagine a great palace with ten floors. In each room, you build the construct of a spell: a combination of language and mathematics, symbolism and geometry, which defines how the spell functions in the world. On the first floor you form the simplest spells, but the ones higher up require greater complexity and stronger attachments. The tenth floor is reserved for only the greatest of archmages.
Why ten floors? Difficult to say, but experimental data clearly show that spells can be sorted into ten bands of power and complexity, aside from simple cantrips. The general consensus is that this reflects something fundamental in the mathematics of our reality, though iconoclastic scholars such as Prof. Foridius argue that it reflects nothing more than an artifact of our base-ten counting system."
— Pathfinder Second Edition, Secrets of Magic