The current debate about filtering child sexual exploitation materials (CSAM) on end-to-end encrypted messaging services, like all previous such debates (of which there were many), mostly ignores this basic point. My point today is that even if we had a magical tool that can tell – with perfect 100% accuracy – that a given image or video contains a depiction of a naked child, or that can – with perfect 100% accuracy – identify everything depicted in that media file, it would still not be enough to be able to filter out CSAM accurately and without mis-labeling huge numbers of innocent people’s messages as CSAM. So, to establish whether a given media file is CSAM or not, whatever magical technological tool being used has to have access to the context.