The Buddha-nature of a dog I keep seeing the following koan:


  A monk asked Zhàozhōu, "Does a dog have Buddha nature or not?" Zhaozhou said, "Wú".


What does this mean and why is it important?

More specifically: Does "Wú" mean "no" in this context, or does it mean "the question doesn't make sense and therefore cannot be answered"? I've seen both interpretations, which one is correct?
We should really have a rule on here not to provide koan answers!  It's so important that you figure them out for yourself!  Written out, the answers become hollow and never approach their true 'meaning'.  That said (and no spoilers follow!)...

Mu does mean no, but it also can translate as blankness, voidness, or absence.  That should ring a bell regarding the concept of emptiness or shunyata.  The koan really doesn't have anything to do with dogs or whether or not they have a Buddha nature.  It has more to do with this idea of emptiness - what it is and the initial experience of it.  The monk in this koan is all caught up in scholasticism and conceptualism.  He's using his small mind to try and understand a concept that it's just not capable of of apprehending.  Zhaozhou's answer is an attempt to shake that monk out of his smaller way of thinking and point him instead to mu - the true essence of Zen.