A spell is a contained, axiomatic system structured around an impossibility. Inconsistencies in the laws of nature create tiny seams or breaches in reality, and spells use sequences of sentient thought and ritual to widen these breaches until they allow for greater and more useful impossibilities to be accomplished. The human mind seems to have been designed specifically not to comprehend such things. Untrained readers oversimplify, gloss over, overcomplicate, or outright deny the impossibility in order to protect their own psyche.
This is more intelligent than what spellcasters do. Spellcasters focus their energies on the impossibility until it breaks something within them, and then they hope that they can comprehend it afterwards.
This breach in reality, once apprehended by the spellcaster’s consciousness, does not cease its resistance. It writhes and spasms inside the caster’s mind, attempting to force itself to be forgotten. Holding onto a spell during this process is extremely painful. After a certain point, spellcasters become used to it. This combination of maintained focus and pain tolerance is what allows more experienced spellcasters to maintain more prepared spells than neophytes.
Finally, the spellcaster performs the necessary rituals of thought and action, which wrench the seam open for an infinitesimal moment, and from the breach emerges the energy-stuff that casters call Bleed. Thought itself shapes Bleed into being; it is understood that it was created as an intermediary matter during the world’s creation, to allow the Fundamental Plane to be shaped by the gods present at that time. Bleed is so volatile that any lapse in concentration in this moment causes the spell to misfire disastrously. Magi of incomprehensible power have burst into cascades of undying cancer, or been swallowed by unnamed gods, only because they mishandled Bleed.
But if they hold fast, adhere to every movement of the ritual, and place every thought in the right place, the spell takes a reliable shape.