As long as the spirit works in the physical body, it cannot as spirit live in its true form. It can, as it were, only shine through the veil of physical existence because as a matter of fact, the thought life of man really belongs to the spiritual world. As it appears in physical existence its true form is veiled. It can also be said that the thought life of the physical man is a shadow, a reflection of the true, spiritual being to whom it belongs.
Source: Rudolf Steiner – GA 9 – Theosophy – Chapter III: The Three Worlds: 4. The Spirit in Spiritland after Death