These are fairly straight forward question to which I have found no answers. Years ago, when I was taking a seminar on Quine, the professor explained Quine's philosophy of science. According to Quine, if I remember correctly, science driven by the principle of having theories with the greatest simplicity possible that cover the highest scope. An example that one of my professor always gave: We could take any outdated scientific theory, say Aristotle's physics for example, and make it work if we add a considerably large number of ad-hoc hypothesis, but the quantity of ad-hoc hypothesis that we may need to add in order to save a scientific theory may increase its complexity to the point that it may be better for it to be replaced altogether.
I believe two conclusions follow from this:
(1) There are no crucial experiments. An experiment never is enough to falsify a theory. This is known as the Duhem-Quine thesis, and it holds that we could change a theory while keeping its "core elements" to adapt it to the results of the experiment that otherwise would falsify the theory.
My professor back then always used the word "core". Scientific theories have a "core" and "exterior" elements, and the theory can be kept keeping its "core" while changing "exterior" elements. The problem is that he could not give a satisfactory answer to what exactly are core and exterior elements in a theory, instead he said I should ask to some professors in philosophy of science, which I did, and I receive no answer since non of them agreed with the Duhem-Quine thesis.
(2) Quine also develops a sort of hierarchy that goes beyond the scientific theory. According to Quine we have "Elements in the world", inside the former we have scientific theories, inside the latter there are mathematical rules, and inside again there are logical rules. This means that changing logical rules would imply changing mathematics and science. In order to create the less possible chaos in the order of things, scientific progress change this from above. Changing the elements in the world seems to have the following meaning: if our scientific theory doesn't apply for some objects in the world we may change the objects in order to keep the scientific theory, for example, if a theory X holds that some elements E should behave in a certain way, but this is shown to be false, elements E', E'', etc. can be constructed of the objects that formerly where known as E, in such way that one of the E's will take the place of the former E. I understand this as a change in the classification of elements required by a theory under new phenomena. It looks like Quine holds a nominalism, but according to Quine it is not a nominalism. How can his thought in this matter be understood if not as a nominalism?