Grammar: assumption or presumption?
If we start with presumption, it means roughly 'having a preconceived position or understanding before the start of a debate, hearing or argument'. Meanwhile, assumption means more generally 'having a preconceived idea or expectation of events'. Subtle, but not impossible to separate: we often hear presumption of innocence (which would not be 'assumption of innocence'); while we generally assume that a 24-hour store will always be open when we go there (we don't 'presume' it).
If we dig a little deeper, an assumption is an idea 'not supported by evidence', while a presumption is something based on more than guesswork, based on more evidence. In the forms assume and presume the waters are murkier and they both generally mean '[I] take it for granted [that]'. But still, the pre- in presume signals the difference. Then there are other meanings of assume (assume a new role, authority etc). Presume can also mean 'taking an arrogant stance, view or position', which is why it is called being presumptuous.
About the author
Jesse Karjalainen is the author of The Joy of English: 100 conversations about the English Language, Cannibal – the language and history of the discovery of the New World, and Roanoke – the language and history of Early Colonial America.