Sentience, killing, & extremes? The Buddha always said killing, no matter the circumstance, is always unskillful &amp; leads to negative Rebirths. 


  In no recorded instance did he approve of killing any living being at
  all. When one of his monks went to an executioner and told the man to
  kill his victims compassionately, with one blow, rather than torturing
  them, the Buddha expelled the monk from the Sangha, on the grounds
  that even the recommendation to kill compassionately is still a
  recommendation to kill — something he would never condone. If a monk
  was physically attacked, the Buddha allowed him to strike back in
  self-defense, but never with the intention to kill. As he told the
  monks,
  
  "Even if bandits were to carve you up savagely, limb by limb, with a
  two-handled saw, he among you who let his heart get angered even at
  that would not be doing my bidding. Even then you should train
  yourselves: 'Our minds will be unaffected and we will say no evil
  words. We will remain sympathetic, with a mind of good will, and with
  no inner hate. We will keep pervading these people with an awareness
  imbued with good will and, beginning with them, we will keep pervading
  the all-encompassing world with an awareness imbued with good will —
  abundant, expansive, immeasurable, free from hostility, free from ill
  will.' That's how you should train yourselves."
  
  — MN 21
  http://zugangzureinsicht.org/html/lib/authors/thanissaro/gettingmessage_en.html


Three questions:

- What determines an animal's sentience?

- If killing is never allowed, how would one deal with a parasite infesting their body, or any other animal as so?

- Would not killing the parasite mean it is self-mortification, &amp; thus an extreme, breaking the Middle Way (as it can hinder the practice of the Dhamma too)?

Please help! Metta to all!
Mr Suminda here expressed well what is included in sentience. 

The working out of the exact results of action &amp; intention is not to be done, as far as i know. 

The consequent origination is without a discernable beginning and the exact results of this or that action is not something that is easy to figure out. 

There is no forumulae or a solver which can simulate two beginningless worlds, arrive at the same states of the world and split the desicion tree to compare outcomes; that would not be possible because if two systems evolve in exact same way with an infinite complexity then it is unreasonable to expect the decision tree to split in and by itself, as a matter of fact there is no decision tree either because world just evolves, it does not stop to think should i go this way or that way, become this or otherwise; it goes according to prior development.

However even if there was a decision tree there is no basis for a split if values are exact and same given that the systems simulate the same exact premises and conditions. This is not like simulating a move in chess or blackjack because these games have determined starting points and a "decision maker".

Therefore one can not evaluate actions in this way, there is no way to input values and simulate outcomes for action A comparing to action B, having solved for which one could say that 'one is better than the other because the better one has exactly this much estimated shorter time to Parinibbana'.

Therefore the exact values are difficult to work out. There are other circumstances where this comes into place, like the Parajika of stealing; "A bhikkhu must have stolen a certain amount for the circumstance to be considered an offence of defeat". Figuring out these values in their exacts &amp; particulars is impossible at this point afaik. 

Nothing else makes sense, the type of thinking; 'If i get maggots i will have found my assasin', is going to lead to inconsistencies and reveal hipocrisy imo.
This is because a person who says he will be devoured by worms rather than break his training, he is saying "lives of others are sacred, i can train even as i am devoured by worms", but at the same time this same person probably goes for walks to exercize his legs where he might step on an animal, drives a car that might kill even a group of people or he causes others to walk and drive even if it is for his training or the training of others, so why sacrifice beings when it is better to train even when devoured by worms?  

It is going to be inconsistent because there is no way to say this life-form goes and this one's are ok to hit with a car on a way to make living or the supermarket. There are all sorts of problems with that thinking, it seems like a person like this would want to sit down and not break the sitting posture until arahantship and then go without food but maybe that is not an option as it might kill the bacteria, beings and worms currently in the body.