The old rebbe explains in the 6th chapter about the side which is the opposite of holiness: just like the godly soul which is holy has ten holy types of powers to do good, so too the animalistic soul has the same powers that can do the opposite of good. Although, the soul itself isn't bad rather it comes from a spiritual light which isn't good but isn't bad, rather it could do either of them. But it's still in clothed in three bad 'garments' so to speak, that express it which are: thought, speech, and action. Therefore it expresses bad. In the above we said that the animalistic soul has ten types of powers for the bad. Now the rebbe explains that three of these powers are in the brain and they 'give birth' so to speak to the other seven powers which are in the heart, and express themselves through feelings, as love hate etc. because the feelings originally come from the brain. A proof to this is: that a small child loves things which aren’t valuable, because his brain cannot understand more valuable stuff. So too his anger arises because of little things and so too he is proud of small things etc.  So, when a person thinks, speaks or does bad so all organs of his body are called impure 'garments', so to speak for the bad.