The Book of Threes
An excerpt from Leabhar na dTress ...
Sentience, or self-awareness among living creatures is what set the whole ecology of magic into motion.
Sentient creatures enforce their will on their environment by simply observing and projecting meaning onto those surroundings. Identifying and naming a creature as a "cat" or a crafted object as a "chair," for example, transforms the thing's existence, giving it a signifier that can be referenced and used later. In this subtle way, sentient creatures are constantly creating and reshaping the world around them.
When the first creature developed a level of awareness that allowed it to identify some things a good (pleasurable) and bad (painful), the first wisps of aetheric energy came into existence.
Aetheric energy, or aether, is the byproduct of conscious thought. All sentient beings innately produce aether. Aether allows sentient creatures to shape how the world around them works on both a conscious and subconscious level.
This reshaping and rechannelling of conditions is what mortals have come to refer to, collectively, as magic.
Aspects of Magic in more depth: