Service in the legions (_militia_) was in theory a burden; exemptions from it were occasionally granted as though it were a troublesome duty,[539] and the citizen who did not present himself for the conscription was sold as a slave[540] across the Tiber.[541] But the treatment of the levy and the feeling of the citizens raised this burden (_munus_) into a privilege (_honor_); it was for this reason that free birth was always required as a qualification for a legionary, and that the ranks were never tainted by the admission of men of servile blood. The Servian census was still the scale by which military service was measured, both in the _legio_ and in the select corps of citizen cavalry (_equites_). The legal duration of service throughout the greater part of the Republic was sixteen, or at the most twenty yearly campaigns (_stipendia_) for the foot soldier, and ten campaigns for the knight. The performance of the _munera_ of tribute and military service required a third duty, which was the condition of both. This was the presence of the citizen who was _sui juris_ at the census for the purpose of registration. All who neglected this duty (the _incensi_) could be sold as slaves across the Tiber.[542] The concepts of the individual as the subject of rights, of their tenure and of their infringement, gave rise to a gradually developed theory of the _jura_ with which the citizen was invested, and the mode in which they might be lost, which plays a large part in the speculations of the jurists. It attached itself to the primitive idea of a _capitis deminutio_, the lessening of status caused by the loss of family rights.[543] Gradually jurisprudence evolved the idea of a _caput_ or personality possessed by every individual independent of citizenship, an idea running parallel with the conception of a law of the civilised world (_jus gentium_) independent of the _jus civile_. From this point of view loss of _citizenship_ could be spoken of as a _capitis deminutio_. There was besides an infringement of personality greater even than the loss of citizenship of which the natural man might be the victim. This was the loss of freedom. These two great derogations of _caput_ were spoken of as _magna capitis deminutio_;[544] but finally a more precise classification gave the following three grades of loss of status:[545]—