Explanation:
The Veda says that God does not exist in the statue (Natasya pratimaa). The statue is only a representative model of God and God does not exist in the inert statue. Similarly, [He does not exist in] inert energy like light etc. The Gita also says that those who worship inert matter and energy (inert five elements) are born as inert objects in the world (Bhutejya yaanti…) [1]. Hence, the medium into which God enters is not inert, but is a living being, which is mainly characterized by awareness. The body of the living being is inert but a living being is mainly characterized by life, mind etc., which is awareness.
Note:
Does God only charge the soul and not the body of the Incarnation?
If you simply take the body, it is inert. But God charges an inert body containing the soul. This means that He only enters a living being. It does not mean that He will not charge the body. If there is a requirement, He will charge the body also. When Krishna lifted Mount Govardhana, God charged the entire body of Krishna. Suppose some devotee wants to touch God directly, then He charges the entire body. Some devotees want to embrace the body of the Lord. Then the Lord charges the body of His Incarnation.
Will God enter animals?
He can enter animals too. But generally, He enters animals only for doing some destruction of evil. Or He may become the Tortoise to lift the mountain [2]. In these cases, He may pervade the soul but may not pervade the body. In fact, there is no part of the body where soul does not exist. Except for hair and nails, awareness is present all over the body. So, He may pervade all over the body, except hair and nails. In the case of Krishna, He lifted the mountain on His fingernail. There God pervaded all over the body of Krishna including the inert nail. Actually, there are two possibilities: either God may Himself pervade or His power may pervade all over the body. Now, in case of animal incarnations, there is no guarantee that God Himself will pervade the animal body; only His power may pervade.
[1] This means that the Gita criticizes the worship of inert items since they are not God and God does not enter them either.
[2] This refers to the tale from the Puranas in which Lord Vishnu took up the form of a giant Tortoise to serve as the bearing upon which the Mount Meru was placed for the churning of the Milk Ocean.
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