Final answer:
Alan is experiencing the Desistance phase of a criminal career, which signifies a move away from criminal activities towards potentially leading a lawful life.
Step-by-step explanation:
According to Alfred Blumstein and colleagues, the phase of a criminal career that Alan, a 40-year-old thief who is considering quitting the criminal lifestyle, is experiencing is known as Desistance. This is the phase where an individual with a criminal background moves away from criminal activity and begins the process of ending their involvement in crime. Desistance typically occurs as people grow older and start to reflect on their lives, contemplating changes that may lead to a more positive and lawful existence. This contrasts from other phases such as Initiation, Exploration, and Stabilization, which denote the beginning, experimentation, and maintenance of a criminal lifestyle, respectively.