Hello!
I figured out something...
With a written form of the spell (parchment or book), its rank is considered as 1 rank lower.
Then a rank 1 spell falls to 0.
In order to cast a spell, your rank in the path needs to be higher or equal.
Then if your rank is 0 (non spellcaster, or spellcaster of other path/tradition), you match!
To conclude, non-casters can cast rank 1 spells if written.
If you go further, if written, they can cast rank 2 spells with an automatic Mishap.
Interesting no?
It grants the Spellcasters to start some trade of magic!