Most Unique

No two of the images are identical. The first image is the only one with a red border, the second has no background, the third has a hexagonal border, and the fourth's sphere has a different texture.

It is purely subjective which of these elements could be considered different enough to rank it "most unique"; however, the fifth image is unique in a totally different way. It is the only image which has no attribute unshared with another. So, interestingly the fifth image is the most unique because it is not unique at all!

This puzzle is based upon one I saw in Scientific American, if anyone has further bibliographic information, please let me know.

Update 12/30/99:

David Shay pointed out that this appeared in the December 1975 issue of Scientific American, which Martin Gardner attributed to Tom Ransom.

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