*This post follows on (loosely) from a previous discussion on **maths and computing** and asks what it really means to ‘prove’ something in each discipline.*

An apocryphal story has an Oxbridge maths don lecturing to a group of undergraduates … After some time completely filling a huge blackboard with heavy calculus – with accompanying commentary, he turns to the class and casually notes, “So then, it’s *clear* that …” (the exact claim isn’t important). As he turns to resume his chalk-work, a particularly bold student enquires, “Excuse me, Professor; but is that really* ‘clear’*?” The don steps back and surveys his work; studying the entire board from top-left to bottom-right, with numerous head and eye movements to-and-fro – even some pointing – to cross-check various parts with each other. After a full five minutes of silent contemplation, he turns back to the students, smiles, announces, *“Yes!”*, and carries on as before.

So who’s defining ‘clear’ here?