Tefillin must be written on the skin of a ritually clean animal. (Shulchan Aruch, Orach Chayim 32:12)
Our thoughts and images are the external garb in which the life of the spirit actually manifests itself….
The skin of the body corresponds to the mind's image-making faculty - the imagination. The imagination is the outer garment of the highest intellect [in Hebrew, "sechel"]. Our thoughts and images are the external garb in which the life of the spirit actually manifests itself in our conscious mind. The three internal lights of the intellect are the Neshama, Ruach and Nefesh. They express themselves in our chochma, bina and daat, the three parts of the mind. These three parts of the mind are the spiritual counterparts of the bones, sinews and flesh of the body. Just as these inner components of the body are covered over by the skin, so the three parts of the mind are "covered over by" and expressed in the imagination.
The skin is the most exterior part of the body. It is outside the flesh and sinews, which give the skin its shape, and they in turn are exterior to the bones. Skin without bones, sinews and flesh would be formless and without purpose. Similarly the thoughts and images in the imagination are empty and valueless unless they are in accord with the higher spiritual truth of the sechel. The imagination is called the "externality of the externality" of the sechel, which is "small" in comparison to mature spiritual knowledge and expanded consciousness, which are called "great".
Our main work of purification and clarification in life is with the images in our minds….
The imagination is the animal faculty in man - even an animal has images of what it wants and what it is afraid of. Our main work of purification and clarification in life is with the images in our minds. We have to strip away the "husks" - the illusions, fantasies, misconceptions, material desires and the like - that are embedded in our way of thinking.
Purification and clarification of the images we think with brings to greater faith. Faith is a matter of the view we take of the world in those areas where true knowledge -sechel - is beyond us. In contrast to true knowledge, faith is our picture of the way we believe thing to be: faith is in the realm of the imagination. To have true faith is to have a picture that corresponds with the essential truth, a sign that the imagination had been developed to perfection (See Likutey Moharan II:8).
As long as the imagination is unpurified, a person is beset by material desires and has all kinds of doubts and questions about faith. He is confused, and his thoughts take him away from G‑d and from attachment to the true tzadikim. The source of all these confused ideas is the Vacated Space, which is where the husks derive from (see #27 and #30 above). Their main hold is in the realm of the imagination.
The reason for writing Torah scrolls, tefillin and mezuzot on animal skin is bound up with purification of the imagination - the "skin" - in order to come to complete faith (we have seen that faith depends on the purity of the imagination). Purification of the imagination is through the spirit of prophecy - the influx of true knowledge from a higher realm (see Likutey Moharan II:8). The Torah as a whole, and the passages written in the tefillin and on the mezuzot, are true knowledge - prophetic spirit. By writing Torah passages on the skin of the animal, the imagination (the skin) is purified. For even though the imagination is the lowest level of consciousness, the expanded consciousness embodied in the tefillin is so great that it has the power to purify even the "externality of the externality" and the "smallness of the smallness".
Adapted by Avraham Greenbaum from Likutei Halachot,drushei tefillin, 34a by Rabbi Nathan of Nemirov
Reprinted with permission from the Breslov Research Institute