All that exists lives on its own element, springs from its own element, and returns to its own element. So earth to earth, water to water, fire to fire, and air to air. Purification means to make a certain object itself; nothing added, nothing foreign attached to it which does not belong to it. These two rules make one understand the process by which the mind could be nourished and purified. The mind is nourished by thoughts and impressions that are harmonious and productive of beauty and which result in satisfaction. For harmony is the nature of the soul, beauty is its source and goal, and by harmony and beauty the mind is nourished, as it is made of harmony and beauty. And the same elements are needed to purify the mind of all undesirable thoughts and impressions, harmony as water and beauty as soap, purifying the mind of all thoughts which are void of harmony and beauty.