The Moon, Lord of GROWTH, represents the outer focus of the person’s attention — that which is grabbing hold of the mind. The Moon is a symbol of the sensory input mind, the seat of the so called lower sheaths of occultism and yoga (mind, life, body), the aspect of our consciousness that makes sense of the flow of life on a daily, moment by moment basis (the feeling function). The Moon ultimately allows one to create emotional security and at its best this translates to a steady mind-feeling state that becomes still enough to reflect the light of the Sun. This means that the Moon shows our capacity to integrate (or reflect) Truth (and all things Solar) into daily life and thus embodied existence.
In the Sufic arrangement the Moon is the vital spirit (ar-rūḥ). This is the life essence that forms the makeup of the so called "astral" body — the vital sheath that forms the blueprint for the physical body — and is a subtle medium between the immortal Spirit and the physical body. If the Sun is symbolic of the center of a circle then the Moon can be likened to its circumference.
“The vital spirit, called ar-rūḥ by analogy with the transcendent Spirit, is what Hindus call prāṇa and alchemists spiritus: it is a subtle modality intermediate between the immortal soul and the body. It is to the Divine Spirit as the circumference of a circle is to its center. This vital spirit is relatively undifferentiated; it includes not only the spatially delimited body but also the sensory faculties with their spheres of experience. Ordinarily man is not aware of it, but in certain states of realization this spirit becomes the vehicle for a diffused spiritual light which may even radiate externally.”
— Introduction to Sufi Doctrine, by Titus Burckhardt