Again, it's not insisting that there must be a natural cause, it is merely a recognition that non-material causes are beyond the scope of science. Such theories are not illegitimate in every conceivable sense of the word, but they are illegitimate for the purposes of science.
First, this would be true if the theory of natural evolution were compelling, which it is not.
Sure it is. You don't get consensus otherwise, and that's what you've got. Anyway, if there's some aspect you feel is inadequate, we can always discuss specifics.
Until a workable mechanism is shown it is merely speculation.
There is a workable mechanism, of course - natural selection is the mechanism which drives evolution.
You will allow the theory of evolution to be displaced only if another scientific theory can be presented.
I really don't think it's too much to ask that scientific theories are, well, scientific. The criteria for being a part of science are the same for everyone, in every field - if you wish to redefine those standards for ID theory or some other variety of creationism, then the burden is rightly on you to show how science is advanced by doing so. Plus, it's not enough to simply have a scientific alternative, you have to have a better scientific alternative. Then the biology classrooms of the nation will be yours, but not until then.
At the same time you are ruling out any non-naturalistic theory as being "unscientific" on the face of it.
Yep. You can have any non-naturalistic theory you like, but you don't get to call it "science" unless it meets the standard for being science. Part of that standard is no elves, fairies, witches, unicorns or big invisible guys in the sky who put their thumbs on the cosmic scales - in short, no non-material entities. Theories that invoke such things can be lots of things, but science isn't one of them. Sorry.
Then the natural sciences should admit the possibility that the origin of life and the origin of the species may be beyond their scope.
Sure it is. You don't get consensus otherwise, and that's what you've got.
Consensus cannot be compelled and that is what the promoters of natural evolution are trying to do. Every scientific theory which has been overturned has first been attacked by a minority, if not by a lone individual. Proper science should not be "I am right now shut up," but rather "let the debate begin." If the evidence for natural evolution is so compelling, why are you afraid of an open debate.
There is a workable mechanism, of course - natural selection is the mechanism which drives evolution.
Again we need to make a distinction, this time between micro-evolution (within one species) and macro-evolution (the formation of a new species). No one will dispute the process of micro-evolution, either natural or through artificial breeding. The various breeds of dogs is proof of that. But in the end, no matter how much you breed dogs, you will always end up with a dog.
The problem is when you take these observations and try to use them to explain macro-evolution. There has yet to be shown a mechanism that will explain how random genetic mutations will produce a new species. This become especially apparent when some of the mutation require multiple and simultaneous genetic mutations or when the intermediate forms are useless and do not enhance an organisms chances of survival.
I really don't think it's too much to ask that scientific theories are, well, scientific.
Part of the problem is that the natural sciences have appropriated to themselves the term scientific. The natural sciences are only a part of the knowledge of man. When the limits of the natural sciences have been met they need to be accompanied by the other areas of human knowledge. The natural sciences need to rediscover the humility to say "we do not know." This does not mean that they should stop trying to discover natural explanations but that they should not presume that there must be one.
Theories that invoke such things can be lots of things, but science isn't one of them. Sorry.
Such theories that go beyond what the natural sciences can properly explain (such as at present the origin of life and the origin of the species) are also not science. The statement that "there must be a natural explanation" is not scientific but only a creedal statement of belief.
And neither is the myth of evolution, something came from nothing, and life came from non-life.
“Evolutionism is a fairy tale for grown-ups. This theory has helped nothing in the progress of science. It is useless.” Louis Bounoure. The Advocate, 8 March 1984, p. 17.
http://www.nwcreation.net/evolutionquotes.html