What is the substantial cause of an instance of consciousness? Context: I'm considered a skeptic of rebirth in my tradition which is the Gelug branch of Tibetan Buddhism. I'm asking this question to help me understand what other traditions think. In my tradition it is believed that rebirth is a semi-obscure phenomena the truth of which can be fully known through reasoning alone. I have a hard time understanding how and do not find any line of reasoning I've heard to be particularly convincing.

The strongest reasoning I've seen others in my tradition give for rebirth is that each instance of consciousness must have a substantial cause. And that brain/matter cannot be that substantial cause because brain and consciousness are fundamentally of two different natures. Therefore, each instance must have been proceeded by a previous instance as its substantial cause leading to an infinite regress/progress back/forward in time.

Here is an excerpt from His Holiness the Dalai Lama's book - Kindness, Clarity and Insight - where he briefly summarizes this reasoning:


  "... the nature of
  the mind is mere luminosity and knowing. Mind is something that has
  the capacity of appearing in the aspect of whatsoever object through
  the force of the object’s casting its aspect to it and is an entity of
  mere clarity and cognition, with a nature of experience. It
  disintegrates moment by moment. However, among its many
  causes—classified into substantial cause and cooperative conditions—it
  must, as an entity of conscious experience, have as its substantial
  cause an immediately preceding cause which is a former moment of consciousness. It is not
  possible for an entity with the character of luminosity and knowing to
  be produced from external material elements as its substantial cause.
  Similarly, an internal mind cannot act as the substantial cause of
  external elements. Since each moment of consciousness requires a
  former moment of consciousness as its substantial cause, there is no
  way but to posit that the basic continuum of mind is beginningless.
  Some specific types of minds [such as desire for an automobile] have a
  beginning and end, whereas other types [such as the ignorance
  conceiving inherent existence] have, in terms of their continuum, no
  beginning but an end. However, neither a beginning nor an end can be
  posited to the mind of luminosity and knowing. Therefore, although
  mind disintegrates moment by moment, its continuum is beginningless."


I believe this reasoning is basically equivalent to Chalmer's Hard Problem of Consciousness. To be clear, I think it is a hard problem for scientific reductionists who believe that consciousness can be reduced to physical matter and energy arranged in a specific way.

On the other hand, it is also equivalent in a different formulation to another famously hard problem: Descarte's famous mind/body problem which assumes the dichotomy of mind/body above and then asks if these are of such fundamentally different natures, then how do they interact? It would seem that positing any mechanism of interaction would betray the original assumption: that they are so fundamentally different that one could never give rise to the other... that they could never touch if you will.

Which leads to the question: what is the substantial cause of an instance of consciousness?


Mind
Matter
Both
Neither
Pineal gland did it!


Would prefer answers with reasoning to help me understand.
In his commentary to Kamalashila's Stages of Meditation, His Holiness the Dalaï-Lama lists 5 types of causes:


  All those phenomena that  are produced at some times but not others
  depend on causes and conditions, and they are of various types. Causes
  are of different types, such as (1) substantial cause, (2) direct
  cause, (3) indirect cause, (4) cause of equal state, (5) concomitant
  cause, and so forth.


He gives a definition of substantial cause:


  [A substantial cause is] A cause that primarily produces its result as its
  substantial continuity; e.g., production of a sprout from a seed.


In 'Kindness, clarity and insight', HHDL says:


  It is not possible for consciousness to be produced from matter as its
  substantial cause. Once consciousness is produced from a former moment
  of consciousness, a beginning to the continuum of consciousness cannot
  be posited.


In addition, in 'Consciousness at the Crossroards', HHDL gives an interesting line of reasoning establishing that matter (form) is not the substantial cause of awareness:


  Specifically, the awareness must have a substantial cause. Now if the
  substantial cause of awareness is matter, then why is it that some
  things have no consciousness, and other things, such as animals, have
  consciousness?


In Pali and Sanskrit, 'substantial cause' is Upādāna. Regardless of contexts where it refers to specifics, literally it translates fuel. The Tibetan for substantial cause is nyer len.

In 'Buddhist Philosophy: Losang Gönchok’s Short Commentary to Jamyang Shayba’s Root Text on Tenets, by Daniel Cozort and Craig Preston, Jamyang Shayba says:


  Because of being a mind, it is established that [another mind]
  precedes it.


And Losang Gönchok comments:


  The subject, a knower [i.e., a mind] just after conception, is
  preceded by a mind that is its substantial cause because of being a
  knower. The subject, the final mind of death of a common being, is
  able to generate the mind that is its impelled object because of being
  a knower having desire, such as a prior main consciousness.



With respect to the ancillary topic (rebirth) it says:


  Because of movement [to activities immediately after birth] and memory
  [of former lives] also, [the existence of former lives is
  established]. Moreover, since a calf who has just been born seeks the
  milk of the teat and since with respect to animals, etc., the mind
  moves to activities of eating grass and engaging in desire without
  being taught, one can understand that this is done due to the
  conditioning of previous lifetimes. Also, memories of former lifetimes
  exist, and matter cannot act as a substantial cause to produce
  consciousness.