While I have full respect for Arthur as a author.. you need to remember.. he writes science fiction.. and its all based around aliens visiting us.. so of course he would have that opinion..
no we dont.. what we have is people making the circles and you guys just cannot believe it..
No.. Occum's Razor actually shows that it is man made not the other way around..
Arthur C Clarke was much more than just a writer of Science Fiction and that was his, no doubt well considered, opinion.
Look at it another way.... man in court accused of criminal damage by creating a crop circle. He could raise all the issues that have been mentioned to prove that he did not do it, could not have done it, and a jury would no doubt find him not guilty. The result would be that the court would conclude that it was done by 'person' or 'persons unknown'. Since humans cannot replicate all the factors that occur in some crop designs we must conclude that some are created by non human 'person' or 'persons unknown'.