I’m defining a positive resolution as a topic that isn’t about whether something should be the case, but rather about what the state of the world. This could be a fact claim/attempting to resolve whether something is factual (e.g., “Global warming is primarily anthropogenic” or “God exists”) or a more normative one (e.g., “Affirmative action benefits minorities”).
I believe that Pro should win such a resolution if they prove that it is probably true. In other words, if the resolution is “God exists,” Con can’t negate by saying “we can’t be certain God exists, and the sentence ‘God exists’ implies a level of certainty.” Pro wins that debate if they show that God probably exists. In other words, the resolution “God exists” is identical to the resolution “God probably exists.”
My reasoning is that almost nothing can be proven to a 100% certainty. Therefore, I think it’s reasonable to assume that if someone makes a positive claim without attaching a probability to it, their claim is just that it is more likely true than not.