Depression and dementia be just one disease, such as the double face of Janus, which Ovide identified in Chaos? Presented for a long time as a differential diagnosis to be considered in early mnesical disorders, these two intrified conditions remain an enigma for the medical body facing the decline of the elderly. Lacking diagnosis, their etiopathogeny remains uncertain in more than one respect. Are siamoise sisters derived from a single pre-existing organic disease or the consequence of each other: the distress of the denial who becomes aware of his disability or the depression to such an extent that it leads to the foledo? The arguments for each of these hypotheses will be put forward with all the usual precautions, without purporting to require the reader to respond unequivocally at the end of the journey.