Because it wasn't merely that the trees were speaking to them. It was that the trees themselves were sentient beings, capable of watching their movements. Was it only the trees in this strange wood, or did every tree observe their movements? Had they always been trying to speak to them? There was no way of knowing, either, if the trees were good of bad, if they lived or hatred humans, if they had principles or compassion. They were like aliens, Gansey thought. Aliens that we have treated very badly for a very long time.
Maggie Stiefvater, The Raven Boys
sentience, trees