The reason it is important to establish what I can and cannot know exists is that truth, at least by the definition I use, can only be considered for things that exist. That's not to say I cannot try to learn truths about things I do not know exist, simply that I cannot state those truths as being true until I confirm it does. Take for example, the Higgs-Boson. Theoretical physicists have declared many properties for a hypothetical construct known as the Higgs-Boson, which allows the creation of a possible model for a number of facets of existence. If we prove that the Higgs-Boson exists in some form by detecting it in a way that fits the properties given to it, great. We may have expanded the truth we as humans have access to. If however, we find that one of the properties of the Higgs-Boson construct does not fit with what we observe, we can say that the Higgs-Boson does not exist as defined outside of being a hypothetical construct. The important thing to note here, is that we can, given the tools, demonstrate the existence or non-existence of the non-hypothetical Higgs-Boson. This means that though I have not yet demonstrated that it does or does not exist, it can still be considered relevant until I do so.
On the other hand, take an overly simplified being that has only one property. That property being that no matter what we do, it is impossible to observe it directly or indirectly. Such a being cannot be considered relevant as there is absolutely no difference in our access to the truth whether it exists or not.
So at this point, I'll say that something is relevant if it can either proven to exist, or able to be proven or disproven to exist at some future point, without change from outside our ability to influence.
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