A garment made of wool and linen is something the Bible forbids, yet the instructions for making some garments of the High Priest, like the Breastplate, include such a forbidden mixture. Rav Kook explains that the mixture is forbidden to teach us that while the animal kingdom (wool) and the plant kingdom (linen) are for our human benefit, they're not the same – we must relate to them differently. Not so when it comes to Divine service, there no such distinction exists; everything is created to serve God. To make this point, the garments of the High Priest combine fabrics of both kingdoms.