Definition
A mental bill is a procedural device in equity practice used to bring new parties before the court when the interests at stake in a pending suit have wholly passed from the original parties to different persons by reason of events occurring after the suit was commenced. The defining feature is the substitution of entirely new parties with new interests — the original parties and their interests do not remain before the court. When the interest of a plaintiff or defendant completely determines (ends) and the same property or right vests in another person not claiming under or through the original party, the proper vehicle is a mental bill, not a supplemental bill.
Common Confusion
MENTAL BILL vs. SUPPLEMENTAL BILL. These two equity pleading devices are closely related and frequently conflated in older sources, but they address distinct situations. A supplemental bill is used when the same parties, or parties claiming under them, remain before the court and new facts arising after commencement of the suit need to be introduced — the original interest continues, just supplemented. A mental bill is triggered when the interest itself has wholly transferred to an entirely new person who does not claim under the original party. Put plainly: if the suit's cast of interested parties changes completely due to intervening events, a mental bill is required; if the same people (or their successors-in-interest) remain, a supplemental bill is the appropriate form.
Why It Matters in Research
This term is a creature of historical equity practice and will appear almost exclusively in pre-merger sources — English chancery materials, American equity treatises, and pre-1938 federal court records from before the Federal Rules of Civil Procedure eliminated the formal distinction between law and equity. Researchers should be aware of several navigational points.
First, the term "mental" in this context does not refer to mental capacity or mental health. The etymology is procedural, not psychological. Searching a corpus for "mental" in an equity procedure context without this understanding will produce significant noise.
Second, the distinction between mental bill and supplemental bill is treated with varying precision across historical sources. Black's and Bouvier's agree on the core distinction but trace it through different illustrative scenarios. Black's anchors the definition in Story's Equity Pleading, which remains the most authoritative American treatise source for unpacking the doctrine in depth.
Third, because the mental bill procedure depended on equity courts maintaining separate procedural dockets, it has no direct modern federal counterpart. Rule 25 of the Federal Rules of Civil Procedure — governing substitution of parties — has largely absorbed the practical function the mental bill once served, but without preserving the terminology or the doctrinal structure. State courts retaining separate equity jurisdiction may preserve the concept in older case law, but modern practitioners do not use the term.
Fourth, corpus researchers working in nineteenth-century American equity records will encounter mental bills most often in property and trust litigation, where interests in land or estates could change hands mid-suit through death, conveyance, or operation of law.
Historical Dictionary Support
Black's and Bouvier's converge on the essential definition: a mental bill is appropriate when new parties with wholly new interests appear, as opposed to a supplemental bill where the same parties or same interests persist. Both sources tie the distinction explicitly to the supplemental bill, treating the two as a paired set.
The primary divergence is in framing. Black's emphasizes the event-driven creation of new interests — something happens after the suit commences that generates entirely new parties. Bouvier's sharpens the trigger condition: the interest of the plaintiff or defendant must "wholly determine," meaning it does not merely shift or expand but ends entirely, with the property vesting in someone claiming no title under the original party. Bouvier's source, Hinde's Chancery Practice, grounds this in English chancery procedure and is somewhat more precise on the vesting requirement.
Both entries gesture toward Joseph Story's Equity Pleading as the controlling American authority. Story's treatment at Section 345 et seq. (cited in Black's) provides the fuller doctrinal analysis that neither dictionary entry fully reproduces. Researchers needing to understand how courts actually applied the distinction in litigation should consult Story directly rather than relying on these compressed dictionary summaries.
Neither source addresses how mental bills interacted with statutes of limitation, nor do they cover the procedural mechanics of filing and service in any detail — gaps a researcher relying solely on these entries would need to fill from treatises and primary court records.
Encyclopedia Cross-Reference
No Law Mind Encyclopedia entry directly addresses historical equity pleading procedure. The matching entries identified (Mental Health Parity, Incorporation of the Bill of Rights, VA Education Benefits) share only superficial keyword overlap and are not substantively relevant to this term.