(Latin: nasci, to be born)
A substance not merely existing by itself (per se), but as containing a principle of action or motion.
The terms nature and substance are frequently used as synonyms.
In the concrete a natural unitary whole may be regarded first as an entity existing by itself, not merely as a modification inhering in some other being, in which case it is called a substance; or we may regard the unit as belonging to some specific kind of reality, i.e., as being the kind of thing it is, in which case it is called an essence; or we may consider it as the ultimate complete principle from which all the activities of the unit proceed, and in this sense it is called a nature.
The nature, then, of a thing is the substance or essence regarded as the complete ultimate principle of the activities of a natural unit.
We understand by natural unit (as opposed to artificial combinations of parts), a unit of which the component parts are, of the very intention of their being, parts and not complete entities in themselves.
It is clear from thia that substance, essence, and nature are in the order of natural existence one and the same thing.
Nature in this sense is only mentally distinct from substance.
The term nature is also more broadly applied to the whole collection of single substances or natures.
Thus we speak of universal nature, applying the term first to the universe of corporeal substances, e.g., the laws of nature, and again to the universe of all created things, e.g., the natural order.
New Catholic Dictionary