[IDEM libro octauo decimo ad Plautium. ] §22.3.8.prSi filius in potestate patris esse neget, praetor cognoscit, ut prior doceat filius, quia et pro pietate quam patri debet praestare hoc statuendum est et quia se liberum esse quodammodo contendit.
[THE SAME, in the eighteenth book on Plautius.] If a son denies that he is under the power of his father, the praetor investigates the matter, to the effect that the son must prove it first, because this must be established both in view of the filial duty which he owes to show to his father, and because he contends that he is in a certain sense free.
Ideo enim et qui ad libertatem proclamat, prior docere iubetur.
For that is why he also who claims his freedom is ordered to prove it first.