The unified understanding of the Spheres is still some ways off in the Dark Medieval period. Each fellowship has its own understanding, not just of magic, but of the world and where the will-worker stands in it — what later ages would call the paradigm. This is encapsulated in the fellowship's Foundation. Specific fellowships rarely use that term, however, and also rarely recognize any commonality between their approaches. A member of the Messianic Voices, for instance, is unlikely to recognize anything like Divinity in the practices of the Old Faith, while the pagans would not see the Messianics as having much Spontaneity, even though both use these concepts as their Foundation.
By increasing one's understanding of one's Foundation, and trying to live by its precepts, a mage increases the potency of one's spells. Raising Foundation requires not just intellectual understanding but spiritual insight and mindful action. This also has an effect of subtly changing the mage as well. For example, a Spirit-Talker with high Sensitivity begins to lose hold of the boundary between the material world and the spirit world, while Hermetic masters who have raised their Modus past a certain point become unnaturally precise and controlled in thought and deed, but also extremely hidebound and set in their habits.
While Foundation represents the central principle of magic, the Pillars represent specific ways of utilizing that magic. A mage who raises their Pillars without putting the same effort into raising their Foundation must exert more effort to achieve the same effects.
Foundation is a trait rated from one to five dots. When casting spells, players generally roll Foundation + Pillar as a dice pool. If the intended effect requires more dots in a Pillar than the character has in Foundation, they must spend a point of Quintessence as well.