Srila Bhaktisiddhanta Sarasvati Thakur —
On the nature of jiva

 
"Before acquiring material designations, the living entity is supremely pure. Even though he is not engaged in serving the Supreme Lord, he remains situated in the neutral position of santa-rasa due to his marginal nature. Though the living entity born from the marginal potency does not at that time exhibit a taste for serving the Lord due to a lack of knowledge of self-realization, his direct propensity of serving the Supreme Lord nevertheless remains within him in a dormant state. Though the indirect propensity of material enjoyment, which is contrary to the service of the Lord, is not found in him at that time, indifference to the service of Hari and the seed of material enjoyment, which follows that state of indifference, are nevertheless present within him. The living entity, who belongs to the marginal potency, cannot remain indifferent forever by subduing both devotional and non-devotional propensities. He therefore contemplates unconstitutional activities from his marginal position.

As a sleeping person dreams that he is active in the physical world without actually being involved in activities, when the dormant indifferent living entity of the marginal potency exhibits even a little apathy to the service of the Supreme Lord and situates himself in a neutral, unchanging condition for even a little time, he is infected by impersonalism. That is why the conditioned soul desires to merge in the impersonal Brahman, thus exhibiting his mind's fickle nature. But due to neglecting the eternal service of the Lord and thereby developing the quality of aversion to the Lord, he cannot remain fixed in that position. In this way aversion to the Lord breaks his concentration of mind and establishes him as the master of this world of enjoyment. (From 'Brahmana & Vaisnava')

Concerning the Origin of the Jiva:

Srila Bhaktivinoda Thakur

A Short Presentation on Jiva-tattva

Eleven Points of Discussion on Jiva-tattva