Definition
A condition attached to a contract, gift, bequest, trust, or other legal instrument that the law will not enforce because it falls into one or more of the following categories: (1) it is factually or legally impossible to perform; (2) it requires conduct prohibited by law; (3) it demands something immoral; or (4) it is repugnant to the essential nature of the transaction to which it is attached.
The critical legal consequence of an illegal condition depends on context. In contracts, an illegal condition may void the entire agreement or, if severable, be struck while leaving the remainder intact. In wills and trusts, courts more frequently sever and discard the offending condition, permitting the gift or bequest to pass unconditionally rather than fail entirely. The distinction between these outcomes — void transaction versus stricken condition — is one of the central practical problems the term presents.
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Common Confusion
ILLEGAL CONDITIONS vs. ILLEGAL CONTRACTS: An illegal condition is a term or requirement embedded within an otherwise potentially valid instrument. An illegal contract is one whose central purpose or consideration is unlawful. The distinction matters because an illegal contract typically cannot be enforced at all, while an illegal condition may be severable from a valid underlying transaction. A researcher encountering "illegal condition" in older sources should not assume the entire instrument is unenforceable — that determination depends on severability analysis and the specific doctrine (contract, trust, or testamentary) at issue.
ILLEGAL CONDITIONS vs. IMPOSSIBLE CONDITIONS: Impossibility is one basis for a condition being deemed illegal in the classical sense, but the two concepts are not identical. Impossibility is factual or legal incapacity; illegality proper involves a legal prohibition or moral offense. Historical sources, including both Black's and Bouvier's, bundle impossibility within the definition of illegal conditions, which can obscure this distinction for modern researchers expecting the terms to operate separately.
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Recognized Forms
/SUBTYPES
1. Conditions contrary to law: Require a party to perform an act prohibited by statute, regulation, or common law rule.
2. Impossible conditions: Require performance of something that cannot physically or legally be done. Historically treated as a subcategory of illegal conditions, though modern doctrine often addresses impossibility separately.
3. Immoral conditions: Require conduct offensive to public morals as recognized by law — historically a broader category, now significantly narrowed and court-dependent.
4. Conditions repugnant to the nature of the transaction: Internally contradictory conditions that defeat the very purpose of the instrument — for example, a condition on an absolute fee that purports to convert it into a defeasible estate in a manner the law does not recognize.
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Why It Matters in Research
The bundled classical definition — impossible, contrary to law, immoral, or repugnant — reflects a doctrinal era when these categories were treated as functionally equivalent. Modern law has largely disaggregated them. Impossibility doctrine has its own rules and remedies. Immorality as an independent ground for invalidating a condition has contracted sharply; courts now tend to frame such questions in terms of public policy rather than morality. Researchers using historical sources must recognize that a case decided under "illegal conditions" on immorality grounds may not map cleanly onto current doctrine.
The severability question is the live research issue. When a condition is struck from a will or trust, the beneficiary typically takes free of it. When a condition is struck from a contract, the result depends on whether the condition was the inducement for the bargain. These rules play out differently across the contracts, trusts, and property corpora, and a researcher following citations across those domains without tracking the doctrinal context can be misled.
Cross-corpus alert: Illegal conditions touching compensation arrangements or forfeiture clauses may also implicate tax treatment of amounts received. Income derived under or despite an illegal condition does not escape taxation — see the gross income / illegal income materials for that intersection.
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Historical Dictionary Support
Black's and Bouvier's definitions are nearly verbatim identical, reflecting the standard formulation that dominated Anglo-American legal writing through the nineteenth and early twentieth centuries. Both list four grounds — impossibility, illegality, immorality, repugnance to the transaction — without ranking them or distinguishing their consequences. This uniformity signals that the classical authorities were drawing from a common source tradition rather than independent analysis.
What both sources omit is consequential: neither addresses severability in any detail, neither distinguishes between contractual and testamentary contexts, and neither acknowledges that "immoral" conditions present fundamentally different institutional problems than conditions requiring unlawful acts. The historical definitions are useful as a starting taxonomy but should not be treated as a functional guide to outcomes. A researcher relying solely on these entries would have the categories but not the rules for what happens when a condition falls into them.
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Jurisdictional Note
Severability rules for illegal conditions in wills and trusts vary by state, particularly for conditions courts find contrary to public policy (such as conditions on marriage, religion, or family integrity). Contract severability analysis is similarly jurisdiction-specific. Researchers should not assume a uniform national rule on whether an illegal condition voids the instrument or is merely excised.
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Encyclopedia Cross-Reference
*Defenses — Illegality (Contracts Violating Statute or Public Policy)* (The Law Mind Contracts & Commercial Law Encyclopedia)
*Contracts — Legality and Public Policy (Illegal Contracts, Restraint of Trade)* (The Law Mind Contracts & Commercial Law Encyclopedia)
*Gross Income — Illegal Income and Embezzlement* (The Law Mind Tax Encyclopedia)
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