You always depend on an untested hypotheses at some point. They are absolutely unavoidable.

Here's a very simple example:

- How much testing is sufficient? For example, as far as we can tell the laws of physics don't vary over time scales that we can directly observe. This has been true as far as we can tell.

- So far so good: the hypothesis that 'the laws of physics don't change' has been 'tested'. Next question: is that amount of testing sufficient?

- Well, that seems to be an untestable proposition; it's always philosophically possible that things get pulled out from under your feet, and no amount of testing up to time T lets you know with complete certainty what happens for t > T.

Go even one or two steps deeper, and you quickly find yourself resting on either unfounded, untested, or untestable assumptions.