Definition
Latin: "in the nature of things." A phrase used in both common law pleading and civil law to assert that something or someone actually exists — or, in its negative form, that something or someone does not exist and is therefore a legal fiction or nonentity.
In common law pleading, the phrase appears most prominently in its negative construction. To allege that a party is not in rerum natura is to assert that the person named in the pleading is fictitious — that no such person exists in reality. This was a recognized ground for a dilatory plea, challenging the action on the basis that the named plaintiff or other party had no real existence.
In civil law, the phrase applies to things rather than persons, addressing whether property or an object has actual physical existence — a prerequisite to being the subject of legal rights or obligations.
Common Confusion
The phrase operates in two distinct legal contexts — persons in common law pleading, and things in civil law — and the negative form is far more common in reported legal usage than the affirmative. A researcher encountering in rerum natura in a pleading context should not assume the civil law meaning applies, and vice versa. The phrase also superficially resembles in rerum natura esse (to be in the nature of things), which appears in philosophical and natural law writing; that usage carries no specific pleading significance and should not be conflated with the technical pleading application.
Why It Matters in Research
This term is most likely to appear in historical pleading records, treatise commentary on dilatory pleas, and civil law materials dealing with property classification. Several research traps apply.
First, the negative construction — not in rerum natura — does the real legal work. Positive uses of the phrase are largely philosophical or contextual; the actionable pleading meaning almost always involves the negative. When scanning historical sources, a researcher should search for both forms.
Second, Blackstone's Commentaries (3 Bl. Comm. 301) is the anchor citation for the pleading application. Any case or treatise citing this proposition traces back to Blackstone, which means the underlying authority is concentrated in a single source. Researchers evaluating the weight of authority should account for this.
Third, the civil law application connects to the Institutes of Justinian (Inst. 2, 20, 7), specifically in the context of legacies and whether a thing bequeathed exists. This is a distinct doctrinal lineage from the common law pleading usage and appears in entirely different corpus materials — civil law treatises, Roman law commentaries, and Romanist scholarship — rather than in pleading manuals or case reporters.
Fourth, the phrase has historical relevance to questions of corporate and fictitious party pleading. Because corporations and other legal entities are by nature artificial, early pleading practice occasionally raised in rerum natura objections in contexts that prefigure modern questions about standing and capacity. Researchers working on the history of corporate personhood or fictitious party doctrine may find the phrase a useful entry point.
Historical Dictionary Support
The four source dictionaries are in close agreement. All four define the phrase as "in the nature of things" and identify the core pleading application: alleging that a party is not in rerum natura is equivalent to alleging that person is fictitious. All four cite 3 Bl. Comm. 301 as authority, and Black's and Burrill's both note the civil law application to things, citing Inst. 2, 20, 7.
Burrill's entry is the most precise in isolating the practical pleading meaning, making explicit that the phrase functions as a test of real existence: "in existence; not a fictitious person." Rapalje & Lawrence is the most compressed, treating the definition as self-evident and moving quickly to the pleading application. Neither Black's edition adds material beyond what Burrill's captures, though both preserve the civil law reference.
What the historical dictionaries do not address: the phrase's appearance in natural law and jurisprudential writing, where it carries a broader philosophical sense (things that exist by nature as opposed to by legal convention). This usage is common in early treatises and in writers like Grotius and Pufendorf. Researchers should be alert to context — the philosophical usage and the technical pleading usage look identical on the page but serve entirely different purposes.