Existence
is determined solely by perception. Existence itself is merely a notion that you have come to accept, but has been created only by your meager human senses. Trees existing at all is ultimately just an accepted idea, and no matter how much experience you may have with tree felling, and no matter how much scientific evidence we have to suggest that a tree, in fact, make a sound, it is ultimately just
evidence.
The moral of the saying is that you shouldn't necessarily have absolute faith in experience or science.
I mean, people complain all the time about people having absolute faith in God, but accept the idea that the Earth is round without ever finding out for themselves.
It just seems silly to me.