The general purposes of the provisions of this chapter are: 1. To proscribe conduct which unjustifiably and inexcusably causes or threatens substantial harm to individual or public interests; 2. To give fair warning of the nature of the conduct proscribed and of the sentences authorized upon conviction; 3. To define the act or omission and the accompanying mental state which constitute each offense; 4. To differentiate on reasonable grounds between serious and minor offenses and to prescribe proportionate