Discussing extrapolation in Buddhism reminds me of a method used in the astronomical sciences, for detecting the existence of planets that are not immediately observable. There is something called a wobble effect. Basically, when a star can be observed to "wobble", one has to assume that the wobbling has a cause, and that cause is the gravitational pull of a planet (which may itself escape direct observation) that is orbiting that star. Look at the animation on this page:
http://en.wikipedia.org/wiki/Doppler_spectroscopy
If you think about having a bucket full of rocks, and a rope tied to the handle of that bucket, and you try to stand in one place, turning, spinning that bucket of rocks around you, you cannot stand perfectly straight up, because of the pull of the bucket, you will wobble as you spin around. That is essentially a replication of the wobble effect. The point is, given enough observable information, a reasonable assumption can be made about the true nature of what cannot be directly observed.
Likewise, In studying Dharma theory, one can look at the conditions of things as being the results of previous things, and can then extrapolate, eliminating various factors that don't quite add up, and without adding any made-up factors, arrive at a very sound method of reasoning.
Nagarjuna (ca. 150–250 CE) is probably best known Budddhist theoretician when it comes to logical inferences, especially ones he was able to come up with regarding the nature of cognition. And after all, when we are talking about karma and rebirth, we are talking about sequential moments of cognition (of being aware) or at least the sequential occurrence of the
causes of cognition.
So, yes, there is a good deal of extrapolation (making projections or assumptions based on known information) with regard to karma. In Buddhist theory, "known information" includes assertions based on logical inferences made by observation (as with the case of assuming a planet which cannot be detected exists, based on the fact that a star is wobbling).
We can support the theory of karma because:
1. ordinary cause and effect is observable in every day situations, so it is at least feasible that cause and effect occur in situations which are not immediately observable.
2. while cognition as we know it relies on the physical properties of the brain, there is nothing to suggest that the materials of the brain cognitively witness their own activity (you may think "I have an idea", but your brain does not identify itself as a "self being" having an idea). In other words, while an electrically charged mass of water and fat may be needed for thought (and different areas of thought
can be mapped out in the brain), it cannot be show to actually
produce thought.
4. Therefore, while the
experience of awareness requires a brain and sensory organs, the
causes of awareness are not dependent on physical conditions.
Everything in the universe can be put into two categories:
awareness, and objects of awareness.
(and by the way, the fact of being aware can also be an object of awareness)
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EMPTIFUL.
An inward outlook produces outward insight.