The categories of certainty of knowledge are statistical and fuzzy.
In the course of everyday life, I don’t bother conducting experiments to see if gravity is still working (I haven’t received a bill in ages) or the atmosphere is still breathable. I just assume that they are in more or less the same state as they used to be. Occasionally this is wrong, and I am surprised by a terrible humidity or excess pollen or dust, but it saves a great deal of deliberation time which can be used to think about other things, such as whether I have remembered to take the laundry out of the washer.
As I converse with people or bots convincingly pretending to be people (and perhaps, eventually, bots which are people), most statements come with an implicit certainty tag: I am not knowingly lying to you, I am convinced that this is true. This is called basic honesty, and is both valuable and common. But from time to time I want to talk about more speculative things, and so I tag my conversational topics explicitly.
I have come to note that I use the phrase “I am perfectly willing to believe –” when I think that a proposed theory is consistent with what I know about the universe, but I have very little evidence for it and none against it. I often use it to propose a more moderate version of some explanatory story. For example, consider the proposition that Martin Luther King Junior was assassinated in a plot by the United States government. Do I believe that LBJ signed an order to kill him? I haven’t seen such a document. Is it consistent with my understanding of the workings of the government? I am perfectly willing to believe that some section of the Federal government, probably in an intelligence agency, decided to set it up. I don’t have evidence for it, I don’t have evidence against it, and right now I don’t believe it – but that just means that if more evidence comes to light, I am prepared to accept it.
In general, I don’t believe in big conspiracies, because people do like to talk about what they have been doing. But I am perfectly willing to believe in any number of small conspiracies – limited by number of participants and timespan – because that’s a thing that humans really like to do. Many businesses start out as a small conspiracy to exploit a situation; they stop being conspiracies when they go about their business openly.