In such cases the police have hardly any time or opportunity to obtain a warrant of arrest from court for the offence being serious the offender might escape by the time a warrant is obtained or tamper with material evidence.
The aggrieved party is again not left with the option of filing a complaint; it is the function of the police as the custodian of the welfare of the society to apprehend offenders in cognizable cases, to initiate proceedings and to prevent the commission of such offences. Examples of cognizable offences are murder, culpable homicide, dacoity, etc.
Non-cognizable offences are minor offences where the injury done to the society is comparatively small and as such the aggrieved party is expected to lodge a complaint before criminal proceedings are initiated. Causing simple hurt is, thus, a non-cognizable offence.
The general rule that serious offences are cognizable while the minor offences are non-cognizable is, however, subject to the following exceptions:
Serious offences like forgery under Section 467, I.P.C. or procuring a miscarriage Sections 312 to 314 I.P.C. are non-cognizable offences, the former because of the difficulty for the police in understanding the real significance of the offence on account of its extremely technical nature and the latter because of the undesirability of allowing the police to interfere in such matters on the ground of public policy.
Then even some minor or light offences are made cognizable because of the grave consequences that might ensue if the mischief is not nipped in the bud by taking immediate steps. An instance of this type of cognizable offence is an unlawful assembly.
Proceedings in non-cognizable cases start by means of a complaint but no complaint need be made in a cognizable case.