Each plant is either hot or cold, and so it grows, since the heat of herbs
is their spirit, and the cold is their body. According to their nature, the
herbs flourish and abound in heat or cold. If all plants were hot and none
of them cold, they would be useless. If all plants were cold, and none of
them hot, they would lead to an imbalance, since hot opposes cold, and
cold opposes hot.
Hildegarde von Bingen (1098-1179)