Here is another way to look at this issue.
Siebe said: “There is no car apart from it wheels, chassis etc.”
This simile of Ven. Nagasena can only provide a crude analogy. A human is more than just physical body parts.
One key idea missing in the above analogy is that while a car has no gathi, a human does. These five khandhas are deeply related to each other. For example, our physical body in this life is pretty much compatible with our four other khandhas, unlike the parts of a car.
If we cultivate deva sankhara by engaging in meritorious deeds, speaking accordingly, and thinking accordingly, our four mental aggregates will slowly change in that direction. If, at the end of this human bhava, we have turned our gathi to that of a deva, then the next birth is likely to be a deva. Then our body will be drastically changed to that of a deva.
If one cultivates sankhara appropriate for a dog, then the next bhava is likely to lead to be a “dog bhava” with a body of a dog, and thoughts of a dog, etc.
“Deva gathi” will yield a deva and a “dog gathi” will yield a dog. No matter what gathi we cultivate, the existence that we get is only temporary. We do not know what will happen in future existences (bhava). We need to try to at least get to the Sotapanna stage in this life, to get out of this recurring process.
We are not just our body parts, but our gathi play a key role. While our gathi are is constant flux, they are always there in some form to keep us bound in this rebirth process.
The only way to get rid of the five khandhas is to remove ALL gathi or defilements (dasa akusala, dasa samyojana). This needs to be done in steps though.
First, we need to understand the importance of the worst of the dasa akusala: killing, stealing, too much attachment to sensual pleasures (which includes sexual misconduct), then those done by speech.
Then we need to take steps clean up our vaci sankhara (conscious thoughts) to be compatible with the above.
However, the “apayagami gathi” cannot ever be PERMANENTLY removed until one starts comprehending “anicca, dukkha, anatta“. That is the next step.
To be at least be feed from the apayas, we need to get rid of “apayagami gathi”. That cannot be done without comprehending Tilakkhana.
That is the special message of the Buddha. That has to be learned from the Tipitaka. No one can do that on one’s own, no matter how intelligent one is. So, when Siebe says: “This cannot be learned from books”, that is not correct.
This is because while we can live with a peace of mind by living a moral life, or become a deva or a brahma or a human in the next life by cultivating appropriate sankhara, we have no control of what may happen in the future lives.
If we get born into a “bad environment” and start cultivating sankhara of an animal or even a “hell being” by joining a gang of killers, then the next birth could in an apaya. In fact, any living being existing right now has been through all those realms in the past. This is the hardest point to understand for most people.