M. Lentz has dealt with the statistical and clinical study of criminal lunatics, a subject hitherto entirely untouched in Belgium. He shows that no serious attention has been paid to these criminal lunatics or insane criminals, who may be classed as insane, abnormal, and vicious. If the limits of these different classes are hard to define, criminological science must nevertheless distinguish them, even artificially. Seeking a criterion to characterise criminal lunatics, properly so called, M. Lentz finds it in their pathogeny, manifest in the two elements cause and effect. But amongst these lunatics occur pretty often offenders who only become insane in the course of their detention. They generally pass unnoticed, and go from the prison to the asylum.