In Greek times it was noted that simple geometrical rules could explain many features of astronomy—the most obvious being the apparent revolution of the stars and the circular shapes of the Sun and Moon. But it was noted that with few exceptions—like beehives—natural objects that occur terrestrially did not appear to follow any simple geometrical rules. (The most complicated curves in Greek geometry were things like cissoids and conchoids.) So from this it was concluded that only certain supposedly perfect objects like the heavenly bodies could be expected to be fully amenable to human understanding. What rules for natural objects might in effect have been tried in the Judeo-Christian tradition is less clear—though for example the Book of Job does comment on the difficulty of "numbering the clouds by wisdom". And with the notable exception of the alchemists it continued to be believed throughout the Middle Ages that the wonders of nature were beyond human understanding.