I don't have a great answer, but I would guess it has something to do with a long and delicate chain of experiences, sense perceptions, and memories that begins at birth and continues to expand as life goes on. These perceptions enter the brain and play off each other, while simultaneously playing off natural drives and impulses. All of this combines to make the "will," which is only part free and part influenced by this chain. The free part of it is self-evident. We choose what specific action we take, but part of what we PREFER is determined by this ever-fluctuating chain. What we do with our preference is wholly our choice, however.

The brain is somewhat like a computer in its analysis of the chain within, and it receives the data and stores it. There is no CPU, though; nothing processes the data accurately (or why would we need psychology?). It is stored in the back of our brain, subtly influencing our actions.

The existence of countless variables in everyday life and behavior means that the chain is affected almost at random. We experience something and immediately we reflect on an older experience, and relate the two according to an emotion we have felt before: all of a sudden there is a memory all of its own, with its own story behind it. If the same thing were to happen to A and A', both could easily end up experience totally different things because of this infinitude of possibility.

A million neurotransmitters fire at the same time in two different brains; they started in the same place, but they ended differently.

The chain begins at birth and its first experiences are its most influential. Psychologists say, I believe, that a person forms his/her personality very young. I say the chain is very delicate because it is similar to a set of domminos -- only more random.

That, in a nutshell, is why A' is a drug addict.

I got a bit carried away on that one.