Full Idea
Two distinct entities cannot explain each other, in the same sense of 'explain'. If the height of the flagpole causally explains the length of the shadow, the shadow cannot explain the flagpole, though it may predict the latter.
Gist of Idea
If the flagpole causally explains the shadow, the shadow cannot explain the flagpole
Source
E.J. Lowe (Individuation [2003], 12)
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.93
A Reaction
This seems related to the question of the direction of time/causation. Some explanations can be benignly circular, as when a married couple have a passion for chinese food. [S.Bromberger 1966 invented the flagpole case].