quote:you are holding the position that everything that exists in this physical world can be observed, or otherwise physically tested?
is that basically what you're saying?
Observed, but not always testable. For istance, the Casimir effect is where matter and anti-matter come into being for a breif amount of time and then anhilate each other. There is no way to predict when this will happen or where this will happen (hence it can't be tested per se), but you can observe it. If you mean testable in a loose sense, then yes, everything in the physical world is testable.
I think that some more clarifications are in order
I assume that when you state:
...do you agree that there are things that exist in this physical world that cannot be quantified (i.e. tested in a lab, swished around in a test tube, examined under a microscope, etc.)?
that even though all your examples refer to lab testing that is not your intent (e.g. you do not intend to exclude astronomical observations, even though a star cannot be tested in a lab, swished around in a test tube or examined under a microscope)
Secondly do you mean to exclude indirect observations, such as cloud chamber tracks ? If so then given that there is no such thing as a direct observation of any external physical object, where do you draw the line and how do you justify it ?
Thirdly do you mean to exclude things that are observable in principle but are not actually observable by us (at least at the present time) ? An example might be a distant, small dark astronomical body. For instance we cannot currently detect any cometary nuclei that are in the region of space referred to as the Oort cloud but we might be able to with improved telescopes - and of course anyone in sufficently close proximity to such a nucleus would be able to detect it with means currently available to us.
Perhaps you'd extend your question a bit. What do you think is not testable or observable, but natural?
Also, do you mean observable in principle (given appropriate sensory or instrumental enhancements) or merely in practice by humans (given limitations on human reason and perception) ?
The frontiers of the latter are subject to change. E.g. bacteria existed before Leeuwenhoek and affected human life, but were not observable in practice before the microscope.
There do appear to be phenomenological limits inherent in nature on (at least direct) observability in principle. Someone mentioned the Casimir effect. The Godel incompleteness theorem and the Heisenberge principle come to mind as well.