Not circular, but
it is based on some fundamental axioms or premises that we have to assume to be true (have "faith" in? ).
For instance, uniformity. That the same things we can see and test here or at one place in the universe, would be the same even if we tested it somewhere else. In other words, that the world works the same way over there as it does here.
Another one is that things in nature are happening because of natural causes or things. If we didn't assume this, we wouldn't be able to test anything.
Also, that we can test things in nature and be able to deduce how nature works from those tests.