This one's got me stumped. Mrs. Byrne lists the following:

thetical adj. -- arbitrary; prescribed; laid down; positive.

Now there's a world of contradictions in those definitions.

But then throw antithetical into the mass, and what you could end up with would be:

anti-arbitrary (prescribed); anti-prescribed (arbitrary); up-in-the-air; and negative or doubtful...

So it would follow that thetical and antithetical could mean, to the arbitrary reader, anything that you want them to mean. Context is all?

I'm hoping tsuwm will clarify this whole thetical mess.

Best regards,
BeWildered