meaning - Is there honour among thieves or not?

I'm not sure which one of these apparently flatly contradictory proverbs I heard first but I have definitely heard both several times. One of them is:

There is honour among thieves.

Another is:

No honour among thieves.

Do they simply mean the opposite of each other, therefore, at least in my mind cancelling each other out, or is there a subtlety here that I am not picking up on? Are they related, perhaps sharing an origin and then somehow becoming inverted?

Answer

The classic proverb holds that, "There is honor among thieves.”¹

The meaning, of course, is the concept of "professional courtesy," that even the disreputable and unethical do - particularly among themselves - adhere to various sorts of moral codes of conduct.²

As to the converse, "no honor among thieves," the meaning is self-explanatory. Curiously, the concept isn't limited to the English-speaking world, as evidenced by this Spanish proverb: Piensa el ladrón que todos son de su condición. (The thief thinks that all are of his condition.)³

^{1 Early nineteenth century. "The Oxford Dictionary of Quotations," Fifth Edition, edited by Elizabeth Knowles (Oxford University Press, Oxford and New York, 2001). Page 612:11.
2 Also: "Honor is sometimes found among thieves." [Walter Scott]
3 http://www.elearnspanishlanguage.com/vocabulary/expressions/ex-proverbs.html}