You’re staring at an MBE Contracts question about a pandemic shutting down a wedding venue, and suddenly you’re drowning in three defenses that all sound the same: impossibility, impracticability, and frustration of purpose. They all involve unforeseen circumstances. They all discharge contractual duties. And on the exam, they’re all sitting there in the answer choices, daring you to pick the wrong one.

Let’s fix that. These doctrines have clear boundaries, and once you understand what each one actually does, you’ll never confuse them again.

The Big Picture: When Performance Becomes Impossible (Or Just Really Hard)

All three of these defenses share a common foundation: something unexpected happened after contract formation that makes the deal fundamentally different from what the parties bargained for. But here’s the critical distinction that the MBE loves to test—impossibility and impracticability excuse the party who cannot perform, while frustration of purpose excuses the party who can still perform but has lost their reason for entering the contract.

That difference matters enormously when you’re reading a fact pattern. Ask yourself: Is the question about someone who can’t do what they promised, or someone who can do it but doesn’t want to anymore because circumstances changed?

Impossibility: When Performance Literally Cannot Happen

Impossibility applies when performance has become objectively impossible after contract formation due to an unforeseen event. Not harder. Not more expensive. Actually impossible.

The elements you need to spot:

  1. Performance has become objectively impossible (not just difficult or unprofitable)
  2. The impossibility arose after contract formation
  3. The impossibility was not the fault of the party seeking discharge
  4. The party seeking discharge did not assume the risk of the event occurring

Classic MBE scenarios for impossibility include:

Death or incapacity of a person necessary for performance. If you hire a specific portrait artist to paint your wedding, and that artist dies before completing the work, performance is impossible. The contract is discharged. But notice the word “specific”—if you contracted with a painting company (not a particular individual), the company’s duty isn’t discharged just because one employee quit.

Destruction of the subject matter. A seller contracts to sell you a specific vintage car. Before delivery, the car is destroyed in a tornado. Performance is impossible, and the seller’s duty is discharged. The key word again is “specific.” If the contract was for a generic 2015 Honda Accord (not “the 2015 Honda Accord in my garage”), destruction of the seller’s particular car doesn’t discharge the duty—they can obtain another one.

Supervening illegality. You contract to import a product that becomes illegal to import due to new federal regulations. Performance is now impossible because it would violate the law. The duty is discharged.

Here’s a hypothetical that tests the boundaries: Seller agrees to deliver 500 bushels of wheat to Buyer. Before delivery, a fire destroys Seller’s entire wheat crop. Is Seller’s performance impossible?

No. Unless the contract specifically required wheat from Seller’s particular farm, this is a contract for generic goods. Seller can purchase wheat elsewhere to fulfill the contract. The duty is not discharged. Seller may have to pay more to obtain substitute wheat, but that’s impracticability (maybe), not impossibility.

Impracticability: When Performance Becomes Unreasonably Difficult or Expensive

Impracticability (sometimes called “commercial impracticability” under UCC §2-615) is impossibility’s more flexible cousin. It recognizes that while performance might technically be possible, unforeseen circumstances have made it so difficult or expensive that requiring performance would be fundamentally unfair.

The elements are:

  1. An unforeseen event has made performance impracticable
  2. The non-occurrence of the event was a basic assumption on which the contract was made
  3. The party seeking discharge is not at fault
  4. The party did not assume the risk of the event

The critical question: What counts as “impracticable”?

Not just more expensive. Courts generally don’t discharge duties because performance costs more than expected. A 10% or even 50% cost increase usually won’t cut it. You need a fundamental change that makes performance radically different from what was contemplated—think a 300% cost increase, or a complete disruption of the expected source of supply.

UCC contracts for the sale of goods have a specific framework for this. Under UCC §2-615, a seller’s delay or non-delivery is not a breach if performance has been made impracticable by an unforeseen supervening event. The classic example: a contract to deliver crops from a specific farm, and the entire crop is destroyed by an unforeseen blight. The seller assumed they’d have the crop to deliver; losing it makes performance impracticable.

But here’s the nuance that trips up test-takers: if the seller can partially perform, they must allocate available goods among customers in a fair and reasonable manner. Impracticability may be partial, not total.

Consider this fact pattern: Manufacturer contracts to sell custom machinery to Buyer for $100,000. After formation, Manufacturer’s sole supplier of a crucial component goes bankrupt, and obtaining the component elsewhere will cost an additional $80,000. Is Manufacturer’s performance impracticable?

Probably not. While the cost increase is substantial, Manufacturer can still perform—it’s just more expensive. Unless the contract contemplated that specific supplier as the only source (unlikely for a component), Manufacturer assumed the risk of needing to find alternative suppliers. The duty is not discharged.

Now change one fact: The component was manufactured exclusively by that one supplier, and no substitute exists anywhere in the market. Now you have impracticability. Performance isn’t just expensive; the basic assumption (that the component would be available) has failed.

Frustration of Purpose: When You Can Perform But Your Reason for Contracting Disappeared

This is the one students mix up most often, because frustration of purpose doesn’t excuse the party who can’t perform—it excuses the party whose reason for wanting performance has evaporated.

The elements:

  1. The party’s principal purpose in making the contract has been substantially frustrated
  2. The frustration resulted from an unforeseen supervening event
  3. The non-occurrence of the event was a basic assumption of the contract
  4. The party seeking discharge did not assume the risk

The canonical example is the Coronation Cases from English law. When King Edward VII’s coronation was postponed, people who had rented rooms along the parade route to watch the procession were discharged from their rental contracts. They could still use the rooms—the landlords could perform just fine. But the entire purpose of renting those specific rooms (watching the coronation) had been frustrated.

Here’s how the MBE tests this: Company rents a large hall for December 15th to hold its annual holiday party. On December 10th, a government emergency order prohibits gatherings of more than 10 people due to a disease outbreak. Company seeks to cancel the rental contract.

This is frustration of purpose, not impossibility. The venue can still provide the hall—their performance isn’t impossible. But Company’s principal purpose (hosting a large gathering) has been substantially frustrated by the government order. If the order was unforeseeable and Company didn’t assume the risk of such regulations, the contract is discharged.

Compare that to: Venue seeks to cancel because the government order prohibits them from hosting events. Now the venue cannot perform. This is impossibility (specifically, supervening illegality), not frustration of purpose.

The Risk Allocation Question: Who Bears the Loss?

Across all three doctrines, courts ask: Did the party seeking discharge assume the risk of this event occurring?

This shows up in several ways:

Express contract terms. If the contract says “Seller assumes all risk of crop failure,” then crop failure won’t discharge Seller’s duty under impracticability. The parties explicitly allocated that risk.

Industry custom. In some industries, certain risks are understood to fall on one party. A construction contractor typically assumes the risk of normal weather delays, so a rainy season won’t excuse performance.

Foreseeability. If the event was foreseeable at the time of contracting, courts often find that the party assumed the risk. You can’t claim impossibility from a hurricane if you formed the contract during hurricane season in a coastal area.

The MBE loves testing this with force majeure clauses. If a contract includes a clause excusing performance due to “acts of God, war, or government regulation,” that clause defines who bears the risk. A party can’t invoke impossibility or impracticability for events covered by a force majeure clause if the clause doesn’t excuse their particular obligation.

Partial Impossibility and Divisibility

Sometimes only part of the performance becomes impossible. The question then becomes: Is the contract divisible?

A divisible contract can be broken into separate parts, each with its own payment. If performance of one part becomes impossible, only that part is discharged—the rest of the contract remains enforceable.

Example: Contractor agrees to build five separate houses for Developer, with payment of $200,000 per house upon completion. After three houses are built, a zoning law prohibits construction on the remaining two lots. The contract is divisible. Contractor’s duty to build the final two houses is discharged by supervening illegality, but Developer must still pay $600,000 for the three completed houses.

If the contract is indivisible (all five houses for a lump sum of $1 million), impossibility as to part of the performance may discharge the entire contract, depending on whether the impossible portion is a material part of the bargained-for exchange.

Temporary Impossibility vs. Permanent Impossibility

Not all impossibility is permanent. If the event making performance impossible is temporary, the duty is generally suspended during the period of impossibility, then revived when performance becomes possible again.

But there’s a limit: if the delay would make performance substantially more burdensome or materially different from what was originally contemplated, the duty is discharged entirely.

Think about a contract to deliver fresh flowers for a wedding on June 1st. A trucking strike makes delivery impossible from May 28-June 3rd. Even though the strike is temporary, the delay makes performance materially different (the wedding will be over). The duty is discharged, not merely suspended.

Contrast that with a contract to deliver office supplies “sometime in June.” A week-long strike in early June temporarily prevents delivery. Once the strike ends, the duty revives—delivery later in June still satisfies the contract.

What Happens After Discharge?

When a contract is discharged by impossibility, impracticability, or frustration of purpose, both parties are excused from future performance. But what about performance already rendered?

Restitution may be available. If one party conferred a benefit on the other before the discharge, they can recover the reasonable value of that benefit to prevent unjust enrichment.

Example: Buyer pays Seller $10,000 as a deposit on a custom machine. Before Seller begins manufacturing, a new law makes the machine illegal to produce. The contract is discharged by supervening illegality. Seller must return the $10,000 deposit in restitution—retaining it would be unjust enrichment.

But if Seller had already partially manufactured the custom machine before the law changed, Seller might recover the reasonable value of the work performed, even though the contract is discharged. The key is whether the partial performance conferred a benefit on Buyer.

How to Spot the Right Defense on the MBE

When you see a fact pattern with changed circumstances, use this decision tree:

First question: Can the party still physically perform what they promised?

Second question (if NO to first): Is performance literally impossible, or just much harder/more expensive?

Third question: Was the event foreseeable? Did the contract allocate this risk? Did the party cause the event?

Fourth question: Is the contract divisible? Is the impossibility temporary?

Let’s apply this to a full hypothetical:

Owner hires Painter to paint Owner’s beach house for $5,000, with work to be completed by July 1st. On June 15th, a hurricane destroys the beach house. Painter sues Owner for breach when Owner refuses to pay.

Walk through it: Can Painter still perform (paint the house)? No—the house no longer exists. Is this literally impossible or just difficult? Literally impossible. Was the hurricane foreseeable? Hurricanes happen, but this specific hurricane wasn’t foreseeable, and nothing in the facts suggests Owner assumed the risk. Did Painter cause the destruction? No.

Conclusion: Painter’s duty is discharged by impossibility (destruction of the subject matter). Owner’s duty to pay is also discharged. Neither party breached.

Now change one fact: The contract required Painter to paint “a beach house” (not “Owner’s beach house”). Now destruction of Owner’s particular house doesn’t make performance impossible—Painter could paint a different beach house. But that’s not what Owner bargained for, so we’d analyze whether Owner’s purpose has been frustrated. Since Owner wanted their specific house painted, and that house no longer exists, Owner’s purpose is frustrated. Both parties are discharged.

Memorization Checklist: What You Need to Know Cold

For impossibility, remember: death/incapacity of necessary person, destruction of specific subject matter, supervening illegality. Performance must be objectively impossible.

For impracticability, remember: unforeseen event makes performance extremely difficult or expensive, but not literally impossible. Most common in UCC contracts when source of supply is destroyed.

For frustration of purpose, remember: the party can perform, but their reason for contracting has been substantially frustrated. Think Coronation Cases—room is available, but the parade isn’t happening.

For all three: check whether the party assumed the risk, whether the event was foreseeable, and whether the contract allocated the risk through express terms.

If you’re building your Contracts outline and want all of these discharge rules organized for active recall, FlashTables covers impossibility, impracticability, and frustration of purpose in Section VII (Performance and Discharge), along with the other ways contractual duties end. The two-column format makes it easy to drill the elements of each defense until you can spot them instantly in an MBE fact pattern.

The Bottom Line

These three defenses aren’t interchangeable, and the MBE knows it. Impossibility is for “I literally cannot do this.” Impracticability is for “I technically can, but it’s become radically different from what we contemplated.” Frustration of purpose is for “I can perform, but my reason for wanting this deal has vanished.”

Master the distinction, learn the elements, and always ask who assumed the risk. Do that, and these questions become points in the bank rather than traps that burn your time.