Toynbee examines humanity’s ability to alter established habits when changing circumstances make them dangerous or obsolete. Drawing on historical comparisons, he distinguishes relatively stable features of human nature from social and cultural practices that people have created and can therefore modify. He considers how accelerating technological and social change has intensified the consequences of institutions inherited from earlier periods.

The book applies this argument to several problems of the modern world. Toynbee contends that political fragmentation and nationalism have become particularly dangerous in the atomic age, creating a need for greater international political organisation. He contrasts this pressure towards political unity with the continuing importance of freedom and diversity in religion. He also considers population growth, urbanisation, mechanisation, affluence and leisure. Across these subjects, his central concern is whether humanity can abandon entrenched habits quickly enough to adapt its institutions to the powers and conditions it has created.