The problem with this statement lies in fully understanding what the term detectable (measurable) means. People will often mistakenly conflate two different definitions: Practically Measurable -- we have the technological means to perform the measurement; and Potentially Measurable -- something is potentially measurable if it ever has any effect whatsoever on matter or energy.
Science concerns itself with the natural world, which, by definition is everything that is potentially measurable. "Supernatural" entities, if they exist, are potentially immeasurable because they never have any effect in our natural world. To decide that such do not exist is not hubris because there is no possible way that such a decision can ever "come back to haunt us" so to speak. If we're right, so what; we can never know and such entities can never do anything in our world anyway. If we're wrong, ditto. If such entities exist, they can't ever have any effect in our natural world, so they can't cause anything. It's really a meaningless question. There's no fence to sit on. Dawkins confidence is not that science will one day be able to answer this meaningless question, but that it can eventually develop the necessary technology to observe and explain any effect that occurs on matter or energy.
The distinction of potential and practical measurability is important to make, and I was mixing them up together earlier. However, it seems to hold for both: if I propose something (let's use the example of 'soul') and do experiments to see if it exists, and find no evidence, it may be that:
a) they exist but I have not discovered the means by which souls affect the forms of matter I'm familiar with. This, I imagine, could be said of a great many things we now measure readily after inventing the right bit of kit to do so and/or understanding how it works, what frequency of EM it is, etc.; x-rays comes to mind;
b) they exist but are outside of the potential realm of science, having no effect on energy-matter. This is the category that you seem to suggest we might as well ignore or make an arbitrary decision about, since it makes no pracitcal difference whether we're right or wrong, and which I feel is important to treat with the recognition that we don't know, and which some say means that they never did exist or could exist (defining, I suppose, existence itself upon what is potentially detectable);
c) they do not exist.
It is also humbling, I think, to recognise that practical and potential immeasurability are, to the (non-)observer, utterly indistinguishable, a valid point made by proponents of 'supernatural' phenomena, and one reason why so many scientific hypotheses are ridiculed at first. Most 'supernatural' phenomena are, in fact, proposed as potentially measurable ones - i.e. not literally as
outside nature, just outside our current understanding, otherwise they would not be searched for. Their proponents are desperately waiting for hard physical evidence, whereupon they can say "I told you so" and we move into a new era in which science recognises psychokinesis or whatever it is, just as we moved into an era in which mass was proportional to how fast a body is moving.
There are those who propose truly supernatural phenomena, outside potential measurement, but there are also two other important categories, the dualist and mentalist positions, in which mind is another kind of existence alongside matter, and in which mind is all there is, respectively. There is, if you like (I know you won't
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d) souls exist but are immeasurable by physical instruments, although detectable within the relevant dimension (mental/spiritual); and
e) only souls (or minds) exist and fool themselves into believing in their physical embodiment and perhaps create physical matter as the result of thought, then miss their own reality by staring at a motionless pointer on a soul-dial.
Personally, I think in many cases (including the existence of souls), all the above are philosophically defensible positions, and scientists defend their measurability-measure of reality while others defend more imaginitive ones. I think that philosophy is immensely important and is actually the often ignored basis of science itself. If someone cannot accept that it is just possible that the universe is dual (or purely mental, or sneezed out of the nose of an intergalactic giant, for that matter), then they miss something important. Like Dawkins, I agree that recognising the beautiful power of ETNS is mind-expanding, but not to recognise
the limits of the principle of emergence is to miss an even greater mental vista. Building blocks never explain building blocks, only what they built, and the great philosophical question of existence is itself, apparently, practically - and perhaps even potentially - immeasurable. Of course, there's always the response,
Big deal, I'm not a cosmologist or
I'm not concerned with ontology or
Philosophy is rubbish.
To say that existence is defined by what is measurable, or that if anything else exists it doesn't matter anyway seem to me like a failure of the imagination, although I can see that they're both practical positions. I can't help thinking that if it doesn't matter if something invisible exists or not, and we make an arbitrary decision to forget about it, dismiss it, it might turn out that it was potentially measurable (since, as I said, we can't tell the difference). It also seems as arbitrary a position as those who believe in God because they won't be punished if they're wrong, and if they're right, they get to go to heaven.
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