The fea­ture which dis­tin­guishes humans from all other species is not lan­guage, the oppos­able thumb, nor our over sized brains but our capac­ity for delu­sion. Delu­sion enables both hope, that potent tonic of indi­vid­ual and col­lec­tive per­se­ver­ance, and denial, the anchor of intran­si­gence. We owe our suc­cess to our irra­tional­ity. Yet may well be undone by it.