The police system of all countries is based upon the assumption that the collection of evidence against a suspected criminal is a matter of public interest but that the collection of evidence in his favour is his private concern. It is often said to be more important that the innocent should be acquitted than that the guilty should be condemned, but everywhere it is the duty of the police to seek evidence of guilt, not of innocence.
Source: Power, 1938.
More info.: https://russell-j.com/beginner/POWER18_120.HTM