if from experience we have learned that something that we were sure about with absolute certainty and took as something that we know and not just something we thought or believed, turned out to be false, how can we know that what we think we know now is true and not another illusory mistake?
This approach has been taken to the farthest extent and it has been claimed that you can't really know anything and that even reality can be an illusion and that nothing really exists because even what you think you see or hear or feel with your senses, is nothing but electrical currents in your brain and all of this is just in your head (they show it beautifully in the matrix movie). hallucinations caused by drugs or other things can be looked at as a good example too but the best example happens almost every day, every time you dream you are convinced that what you are dreaming is actually happening and is reality (no matter how unrealistic), until you wake up, and if you ever dreamt that you wake up from a dream only to really wake up moments later from dreaming you are waking up then you get an even stronger sensation of that illusion. Maybe you are just going back and forth from random dreams to a consistent dream.
Who says you are not dreaming right now?
So what you are sure is reality or waking life can, at any moment, turn out to be another dream, hallucination or just an illusion.
BUT, even if all that is true and its all just in your mind, you still need a mind for it to be in. even if what you are experiencing is an illusion, you are still experiencing it and there is still some truth to your experience! and no matter how much you reduce any experience to something unreal that does not exist, there still exists an experience...
So if there is but one thing i can really say that i know is true is that I Exist! (even if everyone can only say it about themselves) but if it wasn't true, i wouldn't be able to say (or think or Experience) that i exist.
We all have this experience of existing and experiencing, whatever that is. So I exist (so existence exists) and that is the most basic thing to start from.
Now reality does seem to be consistent which lead us to the conclusion that there are constants in the universe and if there weren't then we wouldn't be able to advance, have technology and build cars, refrigerators and go to the moon and so on. (yes we did go to the moon)
But again it is claimed that at every point in history we thought we know the truth, only to find out we were wrong, time after time.
For example, years ago everybody knew that the earth was flat and eventually found that it is round. years ago everybody knew that the sun revolved around the earth (and if you said differently you were burned alive by the way) but eventually found out its the other way around. How much of what we think we know now will turn out to be false.
Now to keep that in mind is a good thing as not to become arrogant in regards to what we think we "know", but it is not completely accurate.
If we look at it more closely we realize that what we find out we were wrong about is not totally false but actually becomes (or is realized as) part of the truth. They were not completely wrong in saying the earth is flat because on a small scale looking at the ground under their feet or even up to the horizon the earth looks or is flat, and only when you zoom out to a larger scale you see that the whole planet is round.
Take this hypothetical scenario:
a man is born somewhere and grows up and throughout his life he sees everywhere the effects of gravity. an apple falls down, all the water streams to the lowest point, he feels the pull on his body and everything is basically pulled to the ground an so on...
so then he says to himself, this is a law, everything is going down. Its the truth and i shall call it gravity! and suddenly one day, after "knowing" this truth for years he sees for the first time a bird fly or a balloon going up or something swept by the wind and he falls to his knees and cry: "i was wrong, all my life i was an idiot and believed something false!"
But no, that doesn't make him wrong and it doesn't make gravity not true, it just becomes (or realized as) part of the truth. There are just more aspects in reality that effect objects in different ways.
That is actually how we build our knowledge of what is true and just always discover and realize more aspects about everything we know.
The knowledge we accumulate, scientific or otherwise, is never 100% accurate but it is for the most part constantly increasing in accuracy
