Well, finally, I hope to get back to answering your question. I have been quite busy, completely revising a WEB site, releasing a book, and ... in any case, I really haven't forgotten.
But, to answer the question, we must get a couple of things established. The question is: "how can it be objectively established, reasoning from the evidence (what we can be directly conscious of, including our own natures), that stealing (or theft) is wrong.
We must first establish what we mean by theft or stealing, and, secondly, what we mean by wrong.
I will tell you in very simple terms what I mean by these words. If you agree, you can simply say so, and we will go from there. If you disagree, you can explain how and why, and we can attempt to resolve whatever differences we have along those lines.
My concept of theft is dependent of the concept of property, which I would describe as that which a person has produced by their own effort, or acquired by exchanging what they have produced (bought) or performed some service to obtain.
This assumes all parties are willing to any such exchanges as take place. Loot is not property.
Theft is any means by which someone acquires the property of another without the other's knowing consent, either by using force, the threat of force, or by deception (such as fraud).
By wrong I mean morally wrong. Moral principles pertain only to one class of beings and only one class of actions, human beings and freely chosen actions. They pertain to human beings because they are the only ones that must live by conscious choice. They pertain to chosen actions, because all others are, "caused," by the autonomic nervous system or physical events over which one has no control.
Two things are required, knowledge and ability, before a choice is possible and before it can be a moral one. One must have knowledge, first, of what choices are available (what can I do), secondly, of the possible consequences of the choice (what will happen if I choose this?), and thirdly whether the actions is right or wrong; one must be physically or intellectually able to do what is chosen.
Finally, moral principles are values. All values presuppose an end or purpose; that which advances the purpose is good, and that which inhibits or damages the purpose is bad. There is no such thing as "intrinsic" good. To be good, something must be good to someone for something.
Since these principles are fundamental, we must first either agree or disagree on these, or any further discussion of how something can be established as, "right," or, "wrong," will not be possible.
Hank
I actually do disagree. But, for the sake of discussion, let's proceed using the definitions you've provided. I expect that I'll be satisfied if you can provide sound reasoning from observable evidence to the conclusion that theft (as you have defined it) is wrong (according to your meaning: that theft works against an end or purpose of the thief).
Have at it!