> Im asking to attempt to provide a justification for the commonly held belief that a person has intrinsic value.

OK--and that's rather obviously a nonsensical question, because "intrinsic value" is a vacuous phrase. Value is by definition a subjective property assigned by a "valuer", while "intrinsic" is by definition a property that is independent of other entities. So, you are asking for a justification for the belief that a person has a value assigned to it by a valuer, absent any valuer. Obviously, such a thing doesn't exist due to logical self-contradiction, and thus no justification for it can exist either.

> If you reject that belief and don't believe it is justifiable, why is that the case.

Because it is a logical impossibility.

> If you arbitrarily decide that all why questions are how questions, then you can't really ask a why question can you?

I didn't do that? There are "genuine" "why" questions, they just are nonsensical (when not asking about a demonstrated conscious entity). "Why" has two distinct meanings, one of which is asking for an underlying mechanism, which maps to what I am calling "how" questions (because questions using "why" in that sense can be rephrased using "how"), the other is asking for an underlying intention, those are the "genuine" "why" questions.

So, "Why does the sun rise in the east" is ambiguous. That question might be asking for the mechanism that makes the sun rise in the east, in which case it's a "how" question in disguise that is obviously one that can be examined empirically. Or it might be asking for the intention behind making the sun rise in the east--in which case it makes the unjustified assumption that there is some sort of consciousness behind it that had an intention when making the sun rise in the east, which is why it is a nonsensical question.

Obviously, it is perfectly sensible to ask genuine "why" questions about entities that are known to have intentions, like "Why did you leave the light on?".

> Many ethical systems make the assumption that human happiness has intrinsic value--that it is an inherently good thing.

Arguably, they don't. Obviously, it's difficult to say anything specific about some unspecified ethical systems, but are you sure that you are not confusing the assumption that human happiness has value with the assumption that human happiness has intrinsic value? The assumption that human happiness has value seems common, the assumption that that value is intrinsic, not so much, very much to the contrary: It would seem rather obvious that that assumption is based on the valuation by the philosopher who is making the assumption. Really, it is questionable whether it is even an assumption, given that there is pretty overwhelming and ubiquitous evidence that humans almost universally value their own happiness. While that might not always be discussed explicitly, that often seems more like a technicality to me than like an actual lack of justification.

> They work backwards from that because what is "good" can't be discovered from observation and experimentation.

How else would it possibly be discovered? Or maybe I should ask first: What do you mean by "good"?

> It's definitely one of the basic tenets of Utilitarianism.

Well, I don't necessarily agree, but I also don't think it matters for this discussion, as it's probably a good enough approximation for the purpose.

Also, if you are interested in a longer explanation of why the concept of "intrinsic value" doesn't make much sense, you might be interested in this video: https://youtu.be/2ASKV7Wg2jE