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?
- Pineal gland did it!
Would prefer answers with reasoning to help me understand.