An attempt to commit a crime occurs if a criminal has an intent to commit a crime and takes a substantial step toward completing the crime, but for reasons not intended by the criminal, the final resulting crime does not occur. Attempt to commit a particular crime is a crime, usually considered to be of the same or lesser gravity as the particular crime attempted. Attempt is a type of inchoate crime, a crime that is not fully developed. The crime of attempt has two elements, intent and some conduct toward completion of the crime.
The essence of the crime of attempt in legal terms is that the defendant has failed to commit the actus reus (the Latin term for the "guilty act") of the full offense, but has the direct and specific intent to commit that full offense. The normal rule for establishing criminal liability is to prove an actus reus accompanied by a mens rea ("guilty mind") at the relevant time (see concurrence and strict liability offenses as the exception to the rule).
There is a distinction between "factual impossibility" and "legal impossibility". Factual impossibility is rarely a defense. A standard policing strategy is the use of an agent provocateur to offer temptation to suspected criminals. In some countries, evidence resulting from entrapment is inadmissible. Nevertheless, undercover police officers do sell real or fake contraband such as illegal drugs or guns, as a means of exposing criminal activity. Some consider the use of fake material as a slightly safer way to catch criminals, rather than risk the real contraband falling into the wrong hands. But if there is no actual contraband and the actus reus of the full offense is "possession" of prohibited materials, there can be no criminal possession. Can there be an attempt to possess when, in the circumstances, it was impossible to follow through to commit the full offense? The answer is that mistakes of fact are almost never a defense, as in People v. Lee Kong, and State v. Mitchell, for example.