The definition of the mitzvah of eating matzah according to the opinion of the Ra"sh is considered the intention of the mitzvah. As written in the book Sha'arim (Ktavot Aleph Ramat): "It is necessary to differentiate in matters of mitzvot that require intention, that specifically in a mitzvah where the action itself is the essence of the mitzvah, there the intention is crucial, like eating matzah and blowing the shofar. But where the main aspect of the mitzvah is the result of the action, the intention is not crucial." And since the mitzvah of matzah involves the action of eating it, the mitzvah of intention is part of the essence of the mitzvah.