From any claim and set of evidence to support it, a more complex claim can be formed that is also consistent with the evidence. For example, suppose one is given an enormous body of experimental data to show that the force of gravity is inversely proportional to the square of the distance between any two objects. If the largest distance tested in the experiment is 20 light years, one can postulate that the force of gravity acts as proposed up to 30 light years, and then gets stronger as the distance is increased. Occam's razor is an absolutely necessary part of epistemology. A positive claim is a specific existence claim or claim about the nature of reality.
Positive claims require evidence.
If someone asserts a positive claim and someone else asserts the corresponding negative claim, absent evidence, one must side with the negative claim. If his evidence consisted solely of the argument that you could not disprove his claim, You would be justified in never speaking to him again. If an observer were to watch this exchange and conclude that he had made a claim that he could not prove, but you could not disprove it either so the matter is not settled, the observer would be committing a gross epistemological error. He would be giving equal weight to a positive claim and a negative claim. That is the claim we must side with absent evidence.