The final programme looks at the human brain and the human mind. Are our minds based purely on binary logic, or is there something irrational - something subjective - at the root of reason? We look at ideas in physics, neurology and psychology and suggest that, whatever you think about the existence of God, the ability to believe is a crucial part of living in a fundamentally uncertain world. If we deny this in a quest for rational "certainty", might we be denying the very thing that makes us human?