# How to prove that what we will know isn't true isn't true?

My apologies for the convoluted question. I'm still (kinda) arguing with someone that if we have knowledge then there are facts.

One way I've argued for this, is by saying that what we will know isn't true already isn't true. This seems to amount to point in question, that what we know won't turn out to be untrue.

But I hit a mini stumbling block.

I want to show that otherwise there's no means to know something wasn't true, but am unsure how.

Can anyone show, ideally in the simplest terms imaginable, that:

• if we know that something wasn't true
• then what we will know isn't true already isn't true
• By "what we will know isn't true already isn't true" you basically mean to say that things don't go from being true to being false? (i.e. that if they're false at some point in the future they're already false now.) – Eliran May 28 '16 at 8:25
• yeah, that's about it – user6917 May 28 '16 at 10:10
• The characterization you just accepted from @EliranH is just not true. It has two problems -- one is history, the other is contingency: "The average temperature of the Universe is over 1000K" was true for a while, and is now false, as far as we can tell, forevermore. "It is safe for everyone to burn fossil fuels until we find another power source" was true -- if we had worked hard enough to find another power source, there would have been no problem with that logic. – user9166 May 28 '16 at 17:40
• In that case, the answer is 'mu'. Movements in language games can result in paradigm edits, which do shift facts. But at any given point in time, the players accept a relatively stable set of facts. So there are facts and they are not permanent. "Birds are dinosaurs", "Pluto is a planet" Facts or not? – user9166 May 29 '16 at 2:22
• @jobermark i think you're implausibly / needlessly confusing the issue. they don't mean that what we thought was a fact can turn out not to be, but, as i said, that the facts themselves at a time can later not be a fact at that time. i.e. not a trivial claim, an impossible one – user6917 May 29 '16 at 5:45

I believe the easiest way to logically represent what you are saying is:

(P → ~S) → (F → ~S)

That is, if something is false in the past, it will be false in the future. This assumes that the fact expressly or implicitly specifies the temporal conditions under which it is supposed to be true, thus making its truth value timeless.

The same thing can be represented in predicate logic to express it for any fact whatsoever:

∀x[(P → ~Sx) → (F → ~Sx)]

The negation of this proposition doesn't imply any contradictions, but it does imply conclusions that most people would consider absurd. The following immediately follows from denying it:

Ǝx[F & Sx & (P → ~Sx)]

That might be translated as:

There exists at least one future fact, which was false in the past.

If we consider that propositions are true in virtue of them being consistent with reality, the possibility of the truth value of facts changing could only mean the possibility of reality itself changing. Therefore, the denial of your proposition implies that history would not be fixed, and the following could be the case: Today, it is true that George Washington was the first president of the United States, but ten years from now that might be untrue — not because someone rewrote history but because reality itself for that period somehow underwent a change.

The denial of your proposition would also imply uncertainty with respect to everything. If one of the facts of one event could change, then who's to say that every fact couldn't also change? If reality were in a constant flux, it would be impossible to know anything at all.

• thanks, this was the sorta answer i was looking for, not sure if it'd do the job tho – user6917 May 29 '16 at 10:36
• Am I missing something? – user3017 May 29 '16 at 10:44
• well the ending could be better demonstrated, and i was disappointed that the notation came back as not implying a contradiction. i don't need to be convinced tho – user6917 May 29 '16 at 15:11
• @MATHEMETICIAN. Which ending? The one about uncertainty? – user3017 May 29 '16 at 15:12
• both of the last two paragraphs work from an intuition that may not be shared – user6917 May 29 '16 at 15:14

If it will not be true then, and it does not change its truth value, then it is not true now.

My 2 cents worth:

Knowledge isn't knowledge until the data is sorted out of the mix. Facts then are not simply knowledge, but a function of knowledge and data.