What are the assumptions in science?

In science, when defining a concept or theory, one has to start with what is 'known' or is 'given'. Given a question like, "Does air exist?", one might have to presume that there is a concept that is called 'air', and that it is a 'true' or factual concept. Then one might ask, "What is air?" Then one might need to do a search of the literature available, or an electronic search, for previous definitions of the concept, 'air'. Going as far back as the ancient Greek philosophers, thinking men who originally tackled new concepts, you might find that 'air' is the invisible substance that humans and all living animals 'breathe', and was at the time considered to be 1 of the 5 primary 'elements', i.e. 'earth', 'water', 'air', 'fire', and the absence of all matter, called the 'aether' and is something that is above the ground level of 'earth' and is what makes the wind that when flowing through the trees, makes the move.

So if you can breathe it, i.e. 'air', then it must exist. So depending on the nature of the 'science', there can be a few or many presumptions, or concepts that one already has a knowledge of, and can apply that information, toward defining the concept of which you are desiring to know.