Presumption, like presume, can be used in various senses such as overconfidence or arrogance (his presumption in announcing a date for their engagement without waiting for a confirmation from her, made her really angry), supposition (my judgment is based on the presumption that humans act rationally and in their own interest) and also the basis or reason for presuming something – lets now talk about the presumptions that hold up this proposition. Note the evolution of its meaning from ‘taken for granted (without authority or proof)’ to ‘basis for reason’.
Presumptuous is now generally used in the negative sense to mean ‘marked by over or undue confidence, or impudence’ – his analysis and forecasts are based on extremely poor quality of data and his assertion that growth will touch 12% next quarter is presumptuous.
Presumptive, however, means inferences based. A presumptive cause for an accident thus means the likely or (most) probable cause and presumptive evidence refers to plausible evidence.
Presumption**, Presumptuous*, Presumptive*
See Part I of this post describing the meaning, example and origin of the words presume and consume here.