If a tree falls in the forest...

We found this tree rocking backwards in the forest above Loch Rannoch. It reminded me of the old question: "If a tree falls in the forest and there is no one there, does it make a sound?" This gets to the heart of the definition of sound. Yes the falling tree will make the particles in the air vibrate, But by sound, we surely mean that those vibrations are detected and interpreted. That needs some higher life form. We were there and we observed. If it's fallen since our visit, then it didn't make a sound!