How is it that a word could mean one thing and it's opposite at the same time? Take the word 'cleave' which has two definitions; the more obvious of which is to cut or split. However, it also has another meaning - to adhere or cling.

This sets up all kinds of confusing situations, and eventually renders the word itself absolutely meaningless and unuseable:
A good writer should cleave himself of ambiguity, and cleave to the pursuit of words that cleave themselves from the mediocre mainstream vocabulary.
See. No one could ever agree or disagree with the above statement, because it's virtually impossible to tell exactly what that statement is. And it sounds retarded.

If you take this a step further and use the word 'uncleave', it opens up a veritable literary wormhole of sorts. Since the word is its own opposite, it's impossible to tell which form of uncleave is being used - the one that means uncut or the one that means unstuck. Silicet:
"I thought you told me you cleaved that."
"Well, that's because I did cleave it."
"I can plainly see that it is most certainly uncleaved."
"That's exactly what I'm saying."
"No, you said cleaved."
"Don't be an ass. It's as cleaved as uncleaved can be."
"Listen to me goddammit. That is not cleaved, and if you want to argue about it, we can go outside and I'll cleave you."
"Now that just doesn't make any sense. How bout you go outside and cleave yourself, mothercleaver."

Essentially, a word that is its own opposite can't possibly have any meaning at all, except in context; and context, being merely the perception of the reader, can fluctuate not only among readers, but among readings by the same reader given any number of external and internal events.

I guess what I'm trying to say is, this is bullshit and somebody better do something about it. Pronto.