In 416 BCE, Athens subjected the people of Melos to an act of genocide. All Melian men were executed and all its women and children were enslaved. None of this was unexpected. Melos's oligarchs had been told what would happen if they defied Athens. The Athenians had been perfectly clear that Melos had no alternative. Nonetheless, the oligarchs chose to resist. It was a decision characterised by folly, poor risk assessment, refusal to accept reality, irrational emotional attachment, and breakdown of dialogue. Melos's fate has disturbing parallels with our reluctance or inability to address climate change. We know what causes climate change, we have been warned about it for half a century, we know that its consequences will be ruinous, and we know what to do, so why don't we address it? Our collective failure to act seems to defy explanation; consequently, the problems of climate change appear intractable. Both suppositions are wrong, but they expose deep-seated flaws in how we deal with existential threat. By exploring the origins of our intellectual inheritance, and applying research in fields such as neuropsychology and social theory, we can throw light on our failure, but no monograph has hitherto combined these strands. This study employs historical criticism to investigate the ancient origins of folly, decision theory to show our deficiencies in assessing risk, neuropsychology to understand the importance of our emotional brain, and social identity theory to understand the New Testament roots of some modern attitudes to climate change. The major part of this study examines failure of dialogue. The hermeneutic concept of fusing ancient and modern horizons is discussed and applied to the Melian genocide and, in particular, to the dialogues of Plato ‚Äö- the foundation documents of our dialogic tradition. A broad selection of dialogues is examined to demonstrate the critical importance of Plato's dramatic settings and characterisations. The result is a list of characteristics by which Plato portrays the value, methods, pitfalls and dynamics of dialogue. Some of these points are used to illustrate and augment recent research on climate change communication. One major, and unexpected, finding was that Plato's dialogues signify a retreat from the agora ‚Äö- the political and commercial heart of society. I conclude it is time to reclaim the agora.