Explanation:

The soul is most subtle and it is very difficult to imagine the soul. Thus, the soul can be treated as almost unimaginable, which can stand as the best simile for God. A really unimaginable item does not exist in the world and hence an exact simile for God even in this aspect is impossible. Hence, the most subtle item that can be imagined through a lot of strenuous effort, can be treated as an almost unimaginable item and can be compared to God, since that is the best thing available. From this aspect too, the soul is selected as the best simile for God.