tasya va etasya purusasya dve eva sthane bhavata
idam ca paraloka-sthanam ca sandhyam trtiyam
svapna-sthanam tasmin sandhye sthane tisthan ete
ubhe sthane pasyati idam ca paraloka-sthanam ca

"The jiva-purusa should inquire about two locations, the material world and the spiritual world. The jiva is situated in a third position, called svapna sthana, on the border between these two places. Situated in this marginal place, he can see both the material and spiritual worlds". (Brhadaranyaka Upanisad 4.3.9)

tad yatha maha matsya ubhe kule'
nusancarati purvam ca param caivam
evayam purusa etav ubhav antav
anusancarati svapnantam ca buddhantam ca

"Just as a large fish living in the river wanders from one bank to the other, so the jiva is of similar quality, and is equipped to wander in the Karana waters between the material and spiritual worlds (svapnanta and buddhanta)." (Brhadaranyaka Upanisad 4.3.18)

yat-tatastham tu cid rupam sva-samvedyad vinirgatam
ranjitam guna ragena sa jiva iti kathyate

"Although spiritual and conscious by nature, the jiva lives on the border between the spiritual and material worlds. When it chooses to enter the material world, it becomes contaminated by the modes of nature. That is said to be the spirit soul." (Sri Narada-pancaratra)

Source: http://gosai.com/writings/jivas-and-the-marginal-plane