This contribution aims at reconstructing the system of legal sources as it can be recognised in all its clarity in the De iure praedae. After pointing out that Grotius applied in this work the mathematical method, it is observed that the law has a clear voluntaristic character: 'voluntas universorum ad universos directa lex dicitur'. Even the 'first notion', quoted in Regula I, that is the lex aeterna, has this specific character: 'Quod Deus se velle significarit, id ius est'. Interesting is also the way in which the issue of the ius naturae is developed. Actually, natural law is consistent with Regula I precisely because it is bound to God as a manifestation of his will: 'Dei voluntas non oraculis tantum et extraordinariis significationibus, sed vel maxime ex creantis intentione apparet'. Thus, the creation of the ius gentium primarium and ius gentium secundarium follows the same pattern: it is always a voluntas that splits into a cascade like a 'baroque fountain'. At the end of the same argument we find finally the ius civile: 'Quidquid respublica se velle significavit', as Regula V states. In conclusion, the system of legal sources in De iure praedae is based on premises that differ in many respects from the legal sources of De iure belli ac pacis.