1. 'Nature' refers to the phenomenon that we call 'cause and effect'.
2. 'First cause' on the other hand, is that which gave/gives rise to the phenomenon that we call 'cause and effect'.
3. That which arises through cause and effect is natural.
4. But that which gave rise to (the phenomenon known as) 'cause and effect' itself cannot then be natural.
Tell me if that amends your understanding of what I was saying.
Analogy: Apples are a type of fruit. But that which gave rise to apples cannot then (by pure logic alone) itself be 'apples'.
1. No, 'nature' does not just refer to cause and effect. It refers to a broader subject than that (and includes the external world and ourselves within it). This might also include any other phenomena that we have yet to discover (such as multi-verses, parallel continuums, or quantum mechanical discoveries).
4. I disagree. This is really just an argument from ignorance b/c neither you nor the physicists working on these cosmological questions actually know a sufficient amount about the cause of our local known universe by which to make the determination that it is not natural. In fact, if we did
discover the sufficient/conclusive explanation for the origins of our now experienced phenomenal world then it would
be, just considered, natural - because it would be just one more aspect of our ever growing knowledge about the global 'circle' around us.
The apple analogy doesn't work because the explanation for the derivation of an apple is natural (and that explanation is in fact another apple and the seeds within it!). So too, all of the available evidence we have demonstrates that not only do apples have natural explanations (apples are fruits that came from other fruits, and so on) but so does the origin of all species of plant that we know about - each having a common ancestor with another.
If you're point here was to discuss the very beginning of "the first apple" then the analogy begs the question because in the case of the apple we do in fact have evidence for it's origin (i.e. - common ancestry gives us knowledge of what came before). In the case of our local known universe here, we do not have such information. We have tentative understanding in current cosmology which states that our known universe here
came out of a singularity, and as is yet, we have no way of extrapolating beyond that singularity (which is
the case with the apple). Thus, we cannot say much about a time "prior" to the singular or what it's nature is, and since all of our current knowledge is derived from natural phenomena the lesser assumption (to apply Occams Razor) is to explain the unknown with the known, not the other way around. And when we apply it consistently we must then accept that the natural explanation is not only the likely better one but the only one that has ever been shown to make any sense.