All crimes feature certain elements. Unless the government is able to prove the existence of these elements, it can't obtain a conviction in a court of law. No crime without law. If an act is to be prohibited, a legally authoritative body (such as Congress or state legislature) must spell out in advance what behavior is banned. The U.S. Constitution forbids ex post facto laws, which declare certain acts to be illegal after the behavior occurs. The Constitution also requires that criminal laws be written in precise terms so that a citizen can determine what conduct is illegal. No crime without a criminal act. In American criminal justice, the government punishes people for what they do rather than what they think or say. The First Amendment protects an individual's freedom of thought and speech. The failure to act, however, can be a crime in situations in which an individual has a legal responsibility to do something. Tax laws and child-neglect laws are two examples. Threatening to act and attempting a criminal act can both be criminal offenses. Similarly, conspiring to commit a crime is illegal. Conspiracy statutes criminalize taking steps to carry out a plan to commit a crime. No crime without intent. Intent pertains to the state of mind or mental attitude with which a person does an act. A synonym for intent, mens rea, literally means "guilty mind." The mental design or purpose to commit a crime is the essence of intent. No crime without concurrence. For an act to be a crime, both the act and the intent must occur at the same time.