The concept of reincarnation by itself is pointless, however, without the principles of karma. While I mentioned karma briefly in earlier posts, at this point I'll expand more on the concept.
The spirit or Ka (as described in my posts of early October 2009) is the energy that is the essence of the being - the 'lifeforce', if you will. I picture it as a shimmering cloud of energy that can take any shape and move at incredible speed in and out of various dimensions. It has no permanent molecular structure and can fill any size space or shape (think of the properties of steam). How it thinks and reasons are beyond the topic of this post, but will be discussed in later ones. What matters here is that in the ethereal form, the Ka has total recall all of its experiences in all of its lives both here and on other planes. When it inhabits this physical plane, it's attached to a body and cannot permanently leave until the body dies. I believe it attaches to its body at the moment of conception, but there are those who believe that it doesn't actually attach itself until birth.
Karma is the term that describes the consequences of all of the actions of the Ka over time. To quote Sir Isaac Newton: To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. Or, to quote the Bible: You reap what you sow. That's the basic concept, although there is much more detail to karma than is described so simply in those quotes.
Karma is a solid religious concept, but not in Christianity. It gives a rational explanation to why the world is not 'fair'. One of the difficulties I've always had with most organized Christian religions is the absolute unfairness of life. I couldn't understand a god who decreed that a select few would be born into wealth, while at the same time legions of children would die of starvation; that some would be born with severe physical and/or mental disabilities, while others would be endowed with great musical talents or mathematical genius; that an entire population would be wiped out by a volcanic eruption or devastating earthquake, while others would be spared and flourish. Does God love some more than others? Does He/She play favourites? What justifies the suffering of so many children who could not possibly have done anything in their short lifetimes to deserve God's wrath? Are they being punished for the sins of their parents then? How do you rationalize these "accidents" of birth?
I can only rationalize them through my belief in karma.
In its most simple terms, karma is a type of retribution - payback, if you will - the receiving of good in this life for good deeds done in prior lives - pain experienced in this life for pain inflicted on others in a past life. All of a sudden, life's inequities and injustices start to make sense.
While that is the core of karmic belief, it's only the proverbial tip of the iceberg. If it were truly that simple, one could simply do whatever one wanted in any given life and pass the consequences of those actions on to the next one. In fact, that's exactly what some people do. Fortunately, the Ka pursues its spriritual journey fully cognizent of karmic and reincarnational principles and operates in a manner intended to advance the entity's growth and understanding.