In the Brhad-aranyaka Upanisad (2.2.20 and 4.3.9) it is said:

yathagneh ksudra visphulinga vyuccaranti

evam evasmad atmanah sarvani bhutani vyuccaranti 

"As tiny sparks fly from a fire, so all individual souls have come from the Supreme."

tasya va etasya purusasya dve eva sthane

bhavata idan ca paraloka-sthanan ca

sandhyam trtiyam svapna-sthanam

tasmin sandhye sthane tisthann ete ubhe

sthane pasyatidan ca paraloka-sthanan ca

"A person has two places: the spiritual world and the place where the spiritual world meets another world. There is also a third place, a place of dreams. Standing between them, the soul sees on one side the spiritual world and on the other side the place of dreaming."

This passage describes the individual soul's (jiva-shakti), which can reside in either the spiritual or material worlds (tatastha).

In other words: There are two positions about which the jiva should inquire – the inanimate material world, and the spiritual world. The jiva is situated in a third position, which is a dreamlike condition (svapna-sthana), and is the juncture (tatastha) between the other two. Being situated at the place where the two worlds meet, he sees both the inert world (jada-jagat) and the spiritual world (cid-jagat).

In the Brhad-aranyaka Upanisad (4.3.18) it is also said:

"As a large fish in a river may go to one shore or the other, so a person may go to one world or another. He may go to a world where he is awake, or may go to a world made of dreams." 

In other words : Just as a large fish in a river sometimes goes to the eastern bank and sometimes to the western bank, so the jiva, being situated in karana-jala (the water of cause that lies between the inert and conscious worlds), also gradually wanders to both banks, the place of dreaming and the place of wakefulness

Vrajanatha: How does the Vedanta philosophy define the word "tatastha?"

Babaji: The place where a river's waters meet with the land of the shore is called the 'tata'. The 'tata' is then the place where water meets the land. What is the nature of this 'tata'? It is like the thinnest of threads that runs along the boundary of land and water. A 'tata' is like the finest of lines, so small that the gross material eyes cannot even see it. In this example the spiritual world is like the water and the material world is like the land. The thin line that separates them is the 'tata'. That boundary place is the abode of the individual spirit souls. The individual spirit souls are like atomic particles of sunlight. The souls can see both the spiritual world and the material world created by Maya. The Lord's spiritual potency, cit-shakti, is limitless, and the Lord's material potency, maya-shakti, is gigantic. Standing between them, the individual spirit soul is very tiny. The individual spirit souls are manifested from the tatastha-shakti of Lord Krishna. Therefore the souls are naturally situated on the boundary (tatastha) of matter and spirit).

In other words: The space between the ocean and the land is called tata (the shore), but the place that touches the ocean is actually nothing but land, so where is the shore? The tata is the line of demarcation separating the ocean and the land, and it is so fine that it cannot be seen with the gross eyes. If we compare the transcendental realm to the ocean, and the material world to the land, then tata is the subtle line that divides the two, and the

jiva-sakti is situated at the place where the two meet. The jivas are like the countless atomic particles of light within the sunrays. Being situated in the middle place, the jivas see the spiritual world on one side and the material universe created by maya on the other. Just as Supreme Lord's spiritual sakti on one side is unlimited, maya-sakti on the other side is also very powerful. The innumerable subtle (suksma) jivas are situated between

these two. The jivas are marginal by nature because they have manifested from Krsna’s tatastha-sakti (marginal potency).

Vrajanatha: What is this 'tatastha' nature of the individual souls?

Babaji: Standing between them, the soul can see these two worlds. The 'tatastha' nature of the souls refers to the fact that they must be under the control of one of these two potencies. The actual place of the 'tata' (shore) may change. What was once dry land may be covered with water, and what was once covered by water may again become dry land. If he turns his gaze upon Lord Krishna, the soul comes under the shelter of Lord Krishna's spiritual potency. But if he turns away from Krishna and turns his gaze to the material potency, maya, then the soul is caught in maya's trap. That is what is meant by 'the soul's tatastha nature'.

In other words:  It is the nature that enables one to be situated between both worlds, and to see both sides. Tatastha-svabhava is the eligibility to come under the control of either of the saktis. Sometimes the shore is submerged in the river because of erosion, and then again it becomes one with the land because the river changes its course. In the same way, if the jiva looks in the direction of Krishna – that is, towards the spiritual world – he

is influenced by Krishna sakti. He then enters the spiritual world, and serves Bhagavan in his pure, conscious, spiritual form. However, if he looks towards maya, he becomes opposed to Krishna and is incarcerated by maya. This dual-faceted nature is called the tatastha-svabhava (marginal nature of the soul).

Vrajanatha: Does maya have anything to do with the nature of the spirit souls?

Babaji: No. The spirit souls are completely spiritual. However, because they are atomic in size, the souls are not very strong. That is why maya can dominate them. However, in the soul's nature there is not the slightest scent of maya.