properly "for all one knows"
This expression means a bit like "not only do we not know but we're not in a position to find out."
"With our X-ray and MRI machines broken, for all we know, there are twenty parakeets doing the can-can in the patient's abdomen!" (The speaker does not really think that birds are dancing inside the patient, just pointing out that no one can prove there aren't.)
"We know more about the surface of Mars than we do about the bottom of the ocean. For all we know, there is a real intelligent civilization right under our noses! ...well, a few miles under them." (The speaker thinks there might be intelligent life on the bottom of the ocean but the point of the sentence is still that no one can prove whether there is or isn't.)
"With respect, you are the only person in this room without a computer science degree. For all you know, C++ is the opposite of C--."
"Ha! The other football team has no idea that we have become a much tougher squad to beat. For all they know, it'll be an easy win!"
(Please retain the reference in reprint: http://www.letpub.com/index.php?page=author_education_for_all_we_know)