I was reading A Heart Released: The Teachings of Phra Ajaan Mun Bhuridatta Thera (trans. Thanissaro Bhikkhu) and came across this term, thitibhutam, which is explained as the primal mind. This is from § 6 of the piece:
§6. The root instigator of the cycle of death and rebirth.
sankhara... upadanam... bhavo... jati...
Each and every one of us born as a human being has a birthplace: we have our parents as our birthplace. So why did the Buddha formulate the teaching on sustained conditions only from the factor of unawareness onwards? What unawareness comes from, he didn't say. Unawareness has to have a mother and father just as we do, and we learn from the above line that thitibhutam is its mother and father. Thitibhutam refers to the primal mind. When the primal mind is imbued with delusion, there is a sustaining factor: the condition of unawareness. Once there is unawareness, it acts as the sustenance for the fashioning of sankhara, mental fashionings, together with the act of clinging to them, which gives rise to states of becoming and birth. In other words, these things will have to keep on arising and giving rise to each other continually. They are thus called sustained or sustaining conditions because they support and sustain one another.
Awareness and unawareness both come from thitibhutam. When thitibhutam is imbued with unawareness, it isn't wise to its conditions; but when it is imbued with awareness, it realizes its conditions for what they really are. This is how the matter appears when considered with the clear insight leading to emergence (vutthana-gamini vipassana).
To summarize: Thitibhutam is the primal instigator of the cycle of death and rebirth. Thus it is called the root source of the three (see § 12). When we are to cut the cycle of death and rebirth so that it disconnects and vanishes into nothingness, we have to train the primal instigator to develop awareness, alert to all conditions for what they really are. It will then recover from its delusion and never give rise to any conditions again. Thitibhutam, the root instigator, will stop spinning, and this will end our circling through the cycle of death and rebirth.
I'm hoping someone can further explain this notion of thitibhutam. I'm especially curious about how this concept can be understood without recourse to an originary ground or 'first cause', for to speak of 'primal' is to suggest something primary, and hence, original--this is how I normally understand 'primal'.