From the mouth of babes,

Just been watching QI on BBC1 with the lovely Stephen Fry.
Three very cleaver people discussed the concept of "if a tree falls in forest and there is no-one to hear it, does it make a sound?"

The old points of what we define as sound i.e. a sound it the vibration in the ear and thus no ear, no sound and other such very cleaver points.

Then Jonny Vegas leaned forward and said "you's lot are bangin' on about sound waves an' stuff. By the same token, you see from light an' stuff, so if no-one see's it fall, is the bugger still stood up then?"

Gotta love the genius of a simple mind!