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The Tavarathasutra states that the soul is not detached from karma; it moves immediately and does not remain stationary. The movement is also upward and only until the end of the stage, after which it does not. This is the classical belief. The question arises: how can the formless soul move without the assistance of any material substances like karma or the body? And why does it move upward and not downward or sideways? Answers to these questions are provided here.
The six substances are inherently dynamic like the physical substances. The difference between them is that the material is inherently downward-moving and the soul is inherently upward-moving. When the soul does not move, or if it moves down or sideways, it should be understood that it is due to the association with or bondage of other obstructive substances, namely karma. When the soul is free from the association of karma and its bondage is broken, there is no obstruction remaining, hence the liberated soul must naturally move upward, in accordance with its inherent nature.
In this instance, the term "previous momentum" is pertinent; thus, the liberated soul ascends due to this momentum. Previous momentum refers to the impulse that arises after the previous bondage of karma has been shed. Just as a potter's wheel continues to spin after the stick is removed, due to the momentum that was initially imparted, so too does the soul, upon liberation from karma, continue to ascend based on the momentum from previous karma. Its upward movement does not proceed beyond the ultimate limit. The reason for this is the absence of dharmastikaya. Once the obstructive karmic substance is removed, how does the soul’s upward movement manifest?