impossibility.
1. The fact or condition of not being able to occur, exist, or be done. 2. A fact or circumstance that cannot occur, exist, or be done. 3. Contracts. A factor or circumstance that excuses the performance because (1) the subject or means of performance has deteriorated , has been destroyed, or is no longer available. (2) the method of delivery or payment has failed, (3) a law now prevents performance, or (4) death or illness prevents performance. • Increased or unexpected difficulty and expense do not usually qualify as an impossibility and thus do not excuse performance. 4. The doctrine by which such a fact or circumstance excuses contractual performance. 5. Criminal law. A fact or circumstance preventing the commission of a crime.
Black's Law Dictionary, Second Pocket Edition
A type of defense for a peculiar subset of crime where no crime has actually been committed, but conviction for attempt of a crime may happen, depending on the circumstances. This is one of those things that must be completely understood to be comprehensible, or will otherwise haunt you every night until final exams. Best piece of advice is not to think too hard about it, there are far more worrisome things in this world.
legal impossibility
1. Impossibility due to the fact that what the defendant intended to do is not illegal even though the defendant might have believed that he or she was committing a crime.
Black's Law Dictionary, Second Pocket Edition
Example: Gambling is illegal, for the most part, in the United States, but not in specific places (e.g., Las Vegas, Mohegan Sun, Atlantic City). A man who understands that gambling is illegal, decides to commit this heinous, heinous crime in Las Vegas, not knowing that it is legal to gamble there.
Will he be convicted for the attempt to gamble? Of course not. The fact that the defendant believes he is committing a dastardly crime does not mean he will Go Directly to Jail (Do Not Pass Go, Do Not Collect $200).
For the most part, impossibility can be asserted as a defense against attempt, conspiracy, and solicitation, if the impossibility comes in this form.
2. Impossibility due to the fact that an element required for an attempt has not been satisfied.
Black's Law Dictionary, Second Pocket Edition
Example: A husband attempts to rape his wife.
The common law definition of rape is (1) unlawful carnal knowledge (2) of a woman by a man, not her husband, (3) without her effective consent; while definitions vary from jurisdiction to jurisdiction, they are usually not far from this general definition. (While rape definitions have been changing, albeit slowly, over the years - allowing men and women to be convicted of rape to a certain extent - the removal of the spousal exception has been very slow). The element requiring the victim to be a woman not his wife has not been satisfied, and thus the husband, if brought to trial, will be let off.
For the most part, impossibility can be asserted as a defense against the crime of attempt (in this case, attempted rape).
factual impossibility
Impossibility due to the fact that the illegal act cannot be physically accomplished.
Black's Law Dictionary, Second Pocket Edition
Example: A pickpocket attempts to pickpocket someone, only to find that the pocket is empty.
Will the pickpocket be convicted for attempted thievery? Of course. Just because the pocket's empty doesn't mean he's not as guilty as old Nick.
Example: A man attempts to rape a woman (not his wife), but before he can penetrate her, he goes limp.
Conviction of rape usually the act of vaginal penetration (by a penis) to take place, according to common law. The fact that the man can't physically attempt to commit the crime will not serve as a defense.
In general, factual impossibility can not be asserted as a defense. While the mindset between a person committing a factual impossibility and a legal impossibility is the same (guilty as all hell), the real point of difference between the two is that act being committed is, ultimately, a crime under factual impossibility and not a crime under legal impossibility.
Make sense so far, right?
mixed legal and factual impossibility (hybrid impossibility)
A situation that can be classified as either factual or legal impossibility and, as such, designed to drive people crazy. A bit like the prisoner's dilemma where no matter how it's judged, nobody likes the outcome.
Example: A man hides behind a tree, planning on raping the next woman that walks past his way. He does indeed rape the first woman he sees; however, the woman turns out to be his own wife.
The act itself is not illegal by definitions of common law (it's not rape if you rape your wife, see legal impossibility); but it just so happened that the illegal act could not be physically accomplished due to a fact in the situation (had it been another woman, the act of rape would have certainly happened, see factual impossibility). What is a court to do? If they apply the legal exception to the case, the defendant goes free, which is problematic because he was potentially a danger to society (after all, had some other woman walked past, the defendant would have most certainly been convicted for rape); however, if they apply the factual exception to the case, the defendant gets convicted for no actual crime - rape of a spouse has been specifically excluded by law.
Courts, understandably enough, are reluctant to convict a man who commits no crime and will usually apply the legal impossibility rule to a mixed case and let the defendant go free. It doesn't happen all the time, though, and some bizarre cases (and even more bizarre reasoning) goes through.
People v. Jaffe, 185 N.Y. 497, 78 N.E. 169 (1906) (New York Court of Appeals): Section 550 of NY's Penal Code provides that a "person who buys or receives any stolen property knowing the same to have been stolen is guilty of criminally receiving such property." Jaffe, being a sucker, bought some 20 yards of cloth, believing them to be stolen. As you guess, they were not (well, there's more to this but it's not necessary to actually go into). The courts decided to let him off, as mere intent is not enough if no crime actually occurs.
State v. Guffey, 262 S.W.2d 152: Shooting a deer out of season is illegal. Guffey decided that he was clearly above the law and shot a deer out of season; turns out it was merely a stuffed deer. (Whoops.) Courts decided to let him off because, well, it's not a crime to shoot stuffed deer. The judgment for this case usually elicits a "What the fuck?" from criminal law students shortly before the intellectual crisis sets in and their understanding about impossibility has been totally destroyed.