What must an officer have in order to lawfully arrest someone?

To lawfully arrest someone, an officer must have probable cause. This legal standard requires that the officer have enough facts and circumstances to lead a reasonable person to believe that a crime has been committed, and that the person being arrested is responsible for that crime. Probable cause is necessary to ensure that an arrest is based on more than mere speculation or unparticular hunches; it requires concrete evidence or reliable information supporting the decision to arrest.

In contrast, reasonable suspicion is a lower standard and is sufficient for brief stops and investigative detention but not for making an actual arrest. Witness testimony and consent from the individual may be relevant in certain situations, but they do not fulfill the requirement of probable cause necessary for a lawful arrest. Probable cause is fundamental in maintaining the balance between individual rights and effective law enforcement.