From the nature of things; from the nature of the case; from nature; applied to arguments based on observation or experiment, as opposed to pure reason
Let us hear the Arguments that follow, which are taken, as he saith, ex rerum naturâ.
An index of subjects; used when a work contains multiple indexes, to differentiate from an index of personal names (index nominum), an index of places (index locorum), or an index of words (index verborum)