The Law of Causality means that every effect (like a fallen tree) must have a cause. Come on! If we find a tree fallen over in the forest, then obviously it did fall down. Trees do not grow horizontally along the ground, and they do not snap their trunks in half naturally. Obviously, the tree must have fallen.
This thought experiment is shown worthless by something quite common these days: Asynchronous communication. The question is relative to the senses, that if you were not there at the time something happen did it happen, but I wasn't there when you made this post so did you make this post? Yes, you did. Can I prove that? No. Can I reasonably assert it? Absolutely.
So it is with the world; nothing need synchronize with our senses.
Theres an obvious difference between reality and human observation. Eventually someone will walk past and see that it fell down, but the fact that humans didn't see something doesn't mean that it didn't happen. Did humans see the big bang? Did humans see the dinosaurs? No, but countless people still believe in those events.
The question is asking whether or not the tree actually fell; it is not asking how, or if, we know the tree fell or not. We know that trees grow, and we know that they grow from the ground up. Assuming lack of direct intervention from a person (which would seem to invalidate the premise of nobody having seen it fall), it kind of isn't possible for a fallen tree to have never fallen over. That's what makes it a fallen tree.
We are also given the answer right there in the question; we are told that a tree in a forest has fallen down. Whether anyone ever sees it or not isn't relevant. If someone does, the only good explanation is that it fell over at some point in the past, and that remains the best (and correct) answer even if it isn't the one they come to/use. If nobody does, then the tree still fell over, but nobody ever saw it. Really isn't any kind of issue there.
Of how we may kick and scream.
If a tree falls in the forest, the rules of physics would apply to it, regardless of observation to guarantee the laws of physics were, in fact, still behaving.
If things didn't work outside of human observation, there's reason enough to believe humanity wouldn't have ever had a chance to exist in the first place. As we are dependent upon the continuity and rigid boundaries of our universe to keep existing.
As Neil Degrasse Tyson once said:
"The good thing about science is that it's true whether or not you believe in it."
Just because no one saw it does not mean it didn't fall. Humans are not the only beings and not everything revolves around them. Death and rebirth is how the world goes round not controlled by humans but by the supreme power, be it the universe, a 'god' or pure energy
The first statements says 'A tree in the forest fell down' confirms it. Just because no body saw it that does not mean it never happened. The tree could have fell naturally or from a large storm.
The first line answers the question. Their is no need for people to get caught up in the statement as the answer is right their.
The first sentence says it did. If you can understand that sentence you would know it fell down. If you were not there to hear or see it makes no difference to the first statement. Whether you believe it or not also makes absolutely no difference to the first statement.
If no person was there to testify the event then how would we ever find out about the tree? How do we know that it happened if no evidence would never occur. We would receive no data and we wouldn't even have the change to actually slightly assume that the particular object was there.
If there is no living being to witness an event, there is no possible way to "know" it occurred. For instance if someone told you the ice caps were melting, but you couldn't actually go to the South Pole and see for yourself, how could you believe them? If we are to be rational we must only believe what we see and hear and unless we were around to see it we can just as well imagine the tree we now see on the ground was always there.