You cannot believe what you have been told until you have decided whether it is true or not.
You cannot decide what to believe - you can only decide whether to accept information or not which then leads to you believing it or not.
You obviously cannot believe something without knowing something about it. So you need the information first. If you say that you can decide what to believe, then what you are saying is that you can make that decision irrespective of accepting or rejecting the information. That leads to this:
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To save you some time, I’ll let you know now that is it literally impossible.
You appear to keep shifting the grounds for believing something from “whether it is true or not” to “knowing something about it.”
Clearly, we have different grounds for believing things to be true.
One ground is that we know the proposition to be true in an epistemically convinced sort of way. We KNOW, full stop, that it is true.
Your claim, at least by using the phrase “whether it is true or not” seems to be insisting that we cannot TRULY believe anything unless we have that kind of epistemic certainty.
My first intuition is to respond that such a claim simply isn’t true. We believe, often without thinking or questioning, a great many things told us or implied by the people and events going on around us. We “have faith” that we are being told the truth more often than we actually delve deeply enough into the truth of the matter to KNOW with any kind of certainty at all.
Now, this is where your argument gets dicey.
Your implied claim is that epistemic certainty isn’t what you mean by “know to be true.” What you seem to be insisting is the rather trivial claim that we cannot believe anything unless we believe it, i.e, have some reason, even a reason totally independent of the truth of the matter, to believe it. Well, that is to say something quite irrefutable because it is vacuously true. We believe things because we find them believable for any reason whatsoever, even reasons that have nothing to do with the truth fo what is believed.
Suppose the reason I have for believing a proposition has nothing whatsoever to do with the actual truth value of the proposition, but on something else, entirely – the authority, for example, of the one giving me the information? In that case, I am NOT believing something because I think it is true but BECAUSE I trust the truth bearer.
Now you might insist that such a position would only be held by wide-eyed buffoons and simpletons, but I would argue that is not the case. All that is required is an honest appraisal of own’s own expertise/knowledge, a willingness to defer to an authority on the matter and a developed and accurate sense of the trustworthiness of that person.
In other words, some propositions are believed/believable not BECAUSE we know them to be true but because we trust the ability of the one relating the proposition to us.
Ergo, it is NOT true that “You cannot believe what you have been told until you have decided whether it is true or not.” There exist other reasons for believing what we are told aside from whether we have worked out their truth value for ourselves.