The Principle of Letting Go,
The Principle of Non Cruelty (no inflicting mental pain),
The Principle of Non-Harm (no inflicting physical pain)
After having established in one's mind the meaning of High Working Hypothesis one asks how this point of view about the world gets translated into behavior. It is the function of High Principles to sketch out in broad strokes the basic notion one will need to apply in all one's thinking about all of one's behavior.
High Principles say that if it is all in essence, pain, then one's first and over-arching priority is to dump it. That is the meaning of "Letting Go."
The second and third principles, non-cruelty and non-harm deal with not creating new kamma that will bind one up in the world in the future. The distinction between non-cruelty and non-harm is to indicate that the notion of not creating bad kamma must be applied to the mental as well as to the physical.
The word in Pali is 'sankappa'. This means: 'san' one's own, or 'with'; and 'kappa' means 'fitting' both in the sense of 'fittings' as in equipages and in the behavioral sense of 'proper.' So, whether 'aims' or 'purpose' or 'principles' it is what is fit and propper for one based on High Working Hypothesis.