Arbitrary conception?

What does the Buddha mean by ***arbitrary conception*** in the Diamond Sutra ? > Then the lord Buddha made his meaning even more emphatic by saying: > "Subhuti, when people begin their practice of seeking to attaining > total Enlightenment, they ought to see, to perceive, to know, to > understand, and to realize that all things and all spiritual truths > are no-things, and, therefore, they ought not to conceive within their > minds any arbitrary conceptions whatsoever."