In 1975). Keyfitz suggested that few of the most firmly established facts of demography are based on observations alone – reality is far too messy for that. Instead, it is the interplay of theoretical models and observations that underpins what we know about population – and sometimes it is theory alone that provides the most reliable pointers to the nature of reality. As Keyfitz famously put it, “no model, no understanding” – perhaps the most fundamental of all the facts of demography.