I didn't understand the explanation, and think perhaps the book is wrong. A person is standing on a horizontal floor fells two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces are: I put A) have equal magnitudes and form an action/reaction pair. according to the book, they B) have equal mags and do not form an action/reaction pair. The book explains that b/c the two forces are felt by the same object (the person), it cannot be an action/reaction pair, but doesn't the push of the person lead to the reaction of the push on the person from the earth?