If the co-operation of which we have been speaking is to be complete, it should rest on a more substantial basis than goodwill: it is not sufficient that private citizens should be well-disposed towards their allies, it is necessary also that they should be acquainted with the conditions that govern the mutual relationship. The sphere of police utility is seriously limited by reason of the ignorance which commonly prevails, both as to the executive powers that private persons can assume as citizens, and with respect to the functions that officials may exercise by virtue of their office. In his "History of the Criminal Law," Sir James Stephen thus explains the position. "The police in their different grades are no doubt officers appointed by law for the purpose of arresting criminals, but they possess for this purpose no powers that are not also possessed by private persons.... A policeman has no other right as to asking questions or compelling the attendance of witnesses than a private person has; in a word, with a few exceptions, he may be described as a person paid to perform as a matter of duty acts, which if he so minded, he might have done voluntarily."[220] The law on the subject is further defined by another authority in these words. "If a constable be assaulted in the execution of his office, he need not go back to the wall, as private persons ought to do; and if, in the striving together, the constable kill the assailant, it is no felony; but if the constable be killed it shall be construed premeditated murder."[221] For all practical purposes, however, the only real difference that exists between the powers that are actively made use of by the police, and the latent powers that are vested in every British citizen, is this:--A police officer may arrest (without warrant) if he has a reasonable suspicion that a felony has taken place; a private person cannot arrest unless he has certain knowledge that a felony has actually been committed. Within these limits, and according to our opportunities, the duty of each one of us is clear and inalienable, remaining the same to-day as it was in the time of Queen Elizabeth, when it was written "So that every English man is a sergeant to take the thiefe, and who sheweth negligence therein do not only incurre Evil opinion therefore, but hardly shall escape punishment."[222] CHAPTER XVII POLICE STATISTICS AND PENOLOGY