Meno’s paradox—which asks ‘how will you know it is the thing you didn’t know?’—appears in Plato’s dialogue of the same name. This article suggests that a similar question arises in some supportive relationships. Attention to this question clarifies one condition necessary to justify making a best interests decisions against someone’s will: the decided-for person must be unable to recognise that they have failed to recognise a need. From this condition, two duties are derived: a duty to ensure that someone cannot recognise that they have failed to recognise a need before making a decision against their will; and a duty to provide consensual support to those who have had decisions made against their will, in order to help them to avoid such second-order failures of recognition in the future. The article assesses the Mental Capacity Act 2005 against each of these duties. For each duty, it finds that the Act allows compliance, but does not robustly require it.