During the collaboration of Beeckman and Descartes, the young Frenchman wrote a short treatise on the "paradox of hydrostatics" which comes from Simon Stevin's work. It is certain that Beeckman brought forward the paradox before him. In this note I show its origin in Beeckman's Journal. I follow the sequence of references in his text to Stevin's and find the very theorem of "hydrostatical paradox". I also refer to the importance of hydrostatics for Beeckman, because he thought a hydrostatical pressure model of the gravitation or attraction which is the central problem in his natural philosophy. At the end of their collaboration they thought falling body problem. This problem must give them another problem about the cause of gravitation. I think that in the course of explaining it they came upon the paradox.