There are three central controlling organs in the human body, at least according to Kabbalah: (1) The Heart, (2) the Brain, and (3) the Liver.
In people this breaks down fairly nicely, and the most is said about our circumstances: Our Brain is the seat of the Godly Soul, the Liver is the seat of the Evil Urge or Animal Soul, and the Heart is the war-torn battleground between them. The Heart is the place where everything interesting takes place.
In Angels, their Heart is the seat of the Godly Soul, their Liver would be the battleground, but since there is no seat for their Evil Urge, they don't really have one. Therefore Angels are ruled purely by their Godly Soul.
On the opposite side of the spectrum, Animals, their Animal Soul dwells in their heart and the Brain would be the battleground but there's no seat for the Godly Soul. Therefore Animals are ruled by their Animal Soul. Truly evil people can attain this level where they have all but entirely driven their Godly Soul out of their Brains, and have entirely ceded their Heart to the Animal Soul, this causes a loss of Free Will which was exactly Pharoah's problem. This is why it is said: "His heart was made hard," or more literally "His heart became liver." The Hebrew word used to mean hard in this case is the same word that means "Liver."
This is a grave over-generalization (For one, Angels don't have physical bodies in a classical sense) but it provides useful imagery to explain how People are in the sweet spot that gives us the ability to determine the battle between the forces of Good and 'Evil' which have both taken up residence within us.