The author addresses the question concerning the political grounds for a criminal penalty imposition, which leads to analyze the content of articles 3 and 4 of the Colombian Criminal Code, where the principles of necessity, proportionality and reasoning in the imposition of the penalty are articulated with the purposes that are recognized to it by the Colombian law. By this, it is found that even article 3 restricts the necessity penalty´s application by the judge, to the pre-established cases in the Criminal Code. Considering the distressing character of the penalty and its aptitude for the affectation of fundamental rights, the constitutional argumentation of the principles contained in article 3 must be required to the judge beyond the systematic and formal analysis that he can present about the “tipicidad”, the “antijuridicidad” and the “culpabilidad”. In consequence, in special cases in where it is possible to argue that the imposition of the penalty does not comply with these principles, the judge must be able to abstain from imposing the penalty or even graduate it below the legal minimum. In order to analyze the problem that has been presented, some theories about the purposes and necessity of the penalty are reviewed.