The Fourth Amendment to the United States Constitution provides protection to the common man from arbitrary arrest, and unreasonable searches and seizures. The provision reads as follows: "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized". According to this provision, a police officer must have a 'probable cause' to make an arrest, to search a person or property, and to obtain a warrant from the judge. In other words, there must be sufficient reason for a police officer to arrest a person, or to conduct search and seizure without a warrant. In that case, he must prove that there was probable cause, based on which the action was made. In order to obtain a warrant for search or arrest, the officer must establish the probable cause before the judge. This concept is mainly applied in criminal law, rather than civil law. The concept is defined as follows: "Apparent facts discovered through logical inquiry that would lead a reasonably intelligent and prudent person to believe that an accused person has committed a crime, thereby warranting his or her prosecution". In simple terms, probable cause can be explained as the requirement that has to be satisfied by a police officer so as to make an arrest or search without a warrant, or to obtain a warrant from a judge. The requirement is that, he/she must have adequate reason to believe that the suspect has committed, is committing, or is about to commit a crime. When it comes to search and seizure, the officer must have enough knowledge to believe that stolen goods, contraband, weapons, or any other evidence of crime are hidden in a particular vehicle or property, so as to conduct a search without a warrant. Example I: A police officer finds a person (A) roaming around vehicles parked on the street, in the middle of the night. In that case, the police officer may develop a 'reasonable suspicion' against 'A'. Reasonable suspicion is not a ground for making a warrantless arrest. The officer detains 'A' for a short time and performs a frisk. If nothing is found, there is no ground for an arrest. If 'A' had tools like a hammer, screwdriver, and wrench; and he was getting nervous when questioned, the officer has a probable cause for arresting him. In this case, the officer has sufficient reason to believe that 'A' was about to commit a theft, as he found evidence to prove the same. Example II: A police officer intercepts a car and its driver for speeding. He searches the vehicle, and finds a bag of cocaine hidden beneath the driver's seat. The officer has probable cause for arresting the driver and seizing the vehicle as well as the cocaine, without a warrant. Example III: An officer receives information that a gang of three is planning to rob a bank. He cannot arrest them or seek a warrant on the basis of this information. He follows them and collects enough evidence to prove their intention. He finds them buying masks and gloves, tools like bolt cutters, large bags, etc. The officer seeks a warrant against the gang through his testimony about the information received and the evidence collected. In such circumstances, the chances of obtaining a warrant is very high. So, probable cause is the standard to assess the reason behind a warrantless arrest or search , or a request for search or arrest warrants. From the examples mentioned above, it is clear that it is not based on mere suspicion. In order to make a warrantless arrest or search, or to obtain a search or arrest warrant, a police officer has to establish a probable cause, which must be based on factual evidence. Such evidence can be obtained from different sources. They include observation, expertise, informants, and circumstantial evidence. Observational evidence is the proof collected by a police officer through observation. It includes what he hears, sees, or smells. For example, an officer can arrest those people who were found using illicit drugs. Some officers are trained to detect and identify certain gestures, tools, etc., that may point towards criminal activities. Such expertise is also a source of evidence, that forms the basis of probable cause. Circumstantial evidence is another source. For example, an officer finds a person hiding behind a store in the middle of the night. The person has a bolt cutter and other tools with him. In that case, he can arrest this person, whom he suspects to be a thief. In some cases, informants provide some evidence which can become the probable source. Such information can be received from victims, witnesses, or even informants. Probable cause is not a cause of concern if the defendant consents to the arrest or search. There is no clear-cut method to calculate the amount of evidence required to establish probable cause. It is the judge who has to decide whether there is probable cause to make the arrest or search in a given case. As the standard is flexible and varies from case to case, the most important factor is to attain a balance between the duties of the police and the rights of citizens against arbitrary arrests and unreasonable searches. In general, it can be said that the standard of probable cause is something between 'reasonable suspicion' and 'preponderance of the evidence'. In simple terms, it should not be a mere suspicion, and it need not be that elaborate as 'preponderance of the evidence', the standard of proof required in civil trials. It may also happen that the officer acted in good faith and believed that he had enough reason; but the judge may conclude that there was in fact, no probable cause. In such cases, the evidence becomes inadmissible in court. However, it very important for a police officer to have a valid and strong probable cause, even if the suspect turns out to be innocent. In short, probable cause is a legal concept that can be described as the reason behind: (1) a warrantless arrest or search; or (2) a request seeking a warrant. In Brinegar v. United States, 338 U.S. 160, the Supreme Court held that, "Probable cause exists where the facts and circumstances within the officers' knowledge, and of which they have reasonably trustworthy information, are sufficient in themselves to warrant a belief by a man of reasonable caution that a crime is being committed".