OK - let me clarify the language I've been using. (For brevity, I may speak a bit loosely with certain technical philosophical ideas here, but I'll try to avoid jargon for the most part.) I'm going to spill a good amount of ink here, but that's not because I'm trying very hard to trick you with meaningless words, but on the contrary, I'm trying to spell out very carefully what I'm saying to make it clear that what I'm saying is meaningful, even if you disagree (just like the statement "The earth is flat" is meaningful, though false. But it's not a word game.)

"1+1=2". "The earth is round." "I am sitting on a chair right now." These are all truths.

Truths come in different categories. For example, some truths are mathematical truths. Some truths are truths about the physical world.

Some truths are truths (as opposed to falsities, like "1+1=3") by virtue of other truths being truths. For example, "the internal angles of a triangle add up to 180 degrees" is a truth in virtue of Euclid's fifth postulate ("if a line segment intersects two straight lines forming two interior angles on the same side that are less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles") being a truth. If the latter is a truth, then it must be the case that the former is the truth.

Physical truths are not truths purely by virtue of mathematical truths alone. For example, no amount of mathematical truths grounds the truth that there is an inverse square law describing the force between physical objects. This is why Newton had to make observations about the world to formulate the inverse square law---because the inverse square law is true by virtue not purely of mathematical truths.

You may or may not have contentions at this point with what I've said - put that aside as long as you understand what I'm trying to illustrate - some truths are truths by virtue of other truths, and there are, in principle, some categories of truths such that no truth of one category can be a truth in virtue because of a truth of another category (you can at least understand what I'm saying about physical truths and mathematical truths, even if you have contentions about whether this is actually the case). And this isn't a language game, what I'm saying really does have meaning.

OK - now consider "One ought not murder an innocent person without just cause". This is a truth. This is not merely a bunch of words strung together in a grammatically correct way like "Colorless green ideas sleep furiously". This sentence is an expression of a truth.

Moreover, this is not the same the truth "I believe one ought not murder an innocent person without just cause". This is a truth about my psychological state, expressing something completely different. "One ought not murder an innocent person without just cause" is a truth regardless of whether or not "I believe one ought not murder an innocent person without just cause" is a truth. It is also a truth regardless of whether or not "Most people believe one ought not murder an innocent person without just cause" is a truth. To equate these truths would be to misunderstand what "One ought not murder an innocent person without just cause" means. Just like how "1+1=2" is a truth that is not the same as "I believe 1+1=2". These are completely different. "1+1=2" is a truth regardless of what my belief about it is. Even if I had some strange drive that might somehow be related to reproduction to want to claim "it is not true that 1+1=2", that would have zero bearing on "1+1=2" being a truth. To believe otherwise would be to misunderstand what "1+1=2" means, it's not a truth having to do with my beliefs, desires, motives, psychology, or biology. It is a mathematical truth, which is completely independent from all that. "One ought not murder an innocent person without just cause" is a truth in a very similar way.

In general, ought-truths are not truths by virtue of is-truths alone (of course, ought-truths can be truths of is-truths in conjunction with ought-truths. For example, "I ought not murder Socrates" is a truth by virtue of "Socrates is an innocent person" (is-truth) and "I ought not murder an innocent person" (ought-truth). However, "I ought not murder Socrates" is not a truth by virtue of "Socrates is an innocent person" alone.)

The same applies to the truth "Evolution has sculpted humans for reproduction". This is an is-truth. "I ought not murder an innocent person without just cause" cannot be a truth by virtue of this truth alone. To think that it could would be to misunderstand what "I ought not murder an innocent person without just cause" means. It is not a truth about psychology, biology, history, mathematics, etc. It is a moral truth, and such truths exist independently of truths about the beliefs people have, the psychological states people are in, or biological drives that people have.

Now, you might object that there is no such thing as an ought-truth---"I ought not murder an innocent person" is just the same thing as saying "I believe I ought not murder an innocent person" or "Most believe I ought not murder an innocent person" or "I have bad feelings about murdering innocent people" or something like this. That's fine---but then we're talking about completely different when we talk about "purpose". Purpose in the sense that it is an ought-truth is what I am talking about. I'm not talking about truths about human biology. Those truths are irrelevant---well, not really irrelevant (like I said, ought-truths aren't truths by virtue of is-truths alone, but they can be truths by virtue of is-truths and ought-truths, so there may be relevant is-truths to our purpose), but they are not the whole story.

And to be sure, thus far, when I have been talking about "facts", what I meant was "is-truths", and when I have been talking about "values", what I meant was "ought-truths".

>I don't know. I guess somewhere along the line we became so complex that we managed to hijack our biology. We can focus on short-term pleasure instead of balancing things with long-term satisfaction. I am talking to you out of curiosity.

So this is a good point to respond to that would help clarify what exactly I mean by purpose. Your response here would be very confusing if you meant purpose in the sense that I meant it. It would be as if you said "My destination is the grocery store," and I asked you, "If your destination is the grocery store, why are you walking toward the zoo?", and you said, "I don't know. I'm curious about what's going on at the zoo." Your response here indicates that your destination isn't in fact the grocery store, or at least, "My destination is the grocery store" is not the whole story, it's actually "My first stop is the zoo, and my final destination is the grocery store," or something like this. To believe otherwise would be to misunderstand what "My destination is the grocery store" means. Now of course, a valid response could have been "My destination is the grocery store, but I got distracted." - having a purpose doesn't preclude a weakness of will causing you to take actions not conducive towards that purpose. But you said "I don't know".

And this isn't some weird meaning I've arbitrarily ascribed to the word purpose. No, purpose in this sense is what actually matters. It is not a description of the way in which a bunch of physical stuff is organized.

For some further intuition on this, it might be fruitful to reread my example of murder in a previous comment in this light. Even if it really were the case that murder was natural, that would have no bearing on its immorality. I'm sure you can come up with other examples where what is natural is immoral.