Bart clappingA Buddhist koan is "a story, dialog, question, or statement in the history and lore of Chan (Zen) Buddhism, generally containing aspects that are inaccessible to rational understanding, yet that may be accessible to intuition". One of the most famous koans is this one, as explained by Lisa and Bart Simpson:

Lisa: If a tree falls in the woods and no one's around, does it make a sound?
Bart: Absolutely! [makes sound of a tree falling]
Lisa: But Bart, how can sound exist if there's no one there to hear it.
Bart: Wooooooo… (thanks SNPP)

(The picture to the right is of Bart 'solving' another popular koan, "What is the sound of one hand clapping?")

Now, with all due respect to Lisa, I think the "tree falling" koan is rationally solvable. The answer is yes, there is sound … but there is no perception of the sound. The sound itself (a "series of pressure waves") is the result of the tree falling; our perception of it is when it enters our ear and we interpret it by the process of hearing. Could there be sound in our ear without the sound waves? No, because THAT is the 'sound'. Here's an example: If someone slaps me, and I feel pain, the pain itself is not the slap, the pain is only the perception of the slap. The slap would still exist even if I didn't feel the pain, for example, if I were on pain-inhibiting medication.

Furthermore, the idea that a someone must be present at an event when it occurs for it to truly have happened is ridiculous. We know trees do not (under normal circumstances) fall silently to the ground when they fall; therefore if we find evidence that the tree has fallen, we can conclude that it made a sound. Direct observation is not the only way that we can reasonably know that something is true. For example, we know historical truth by examining the historical evidence. We cannot directly observe, say, Abraham Lincoln being shot, or Columbus "discovering" North America, or even Jesus rising from the dead, but we can still be reasonably certain these historical events occurred by studying the available evidence.