On the standard picture of normative reasoning, one is supposed to weigh, add, or aggregate various pro tanto reasons to conclude whether there is an overall pro toto reason to do something. To do this, one should assign a weight to the reasons for and against, add them together, and conclude whether they are sufficiently strong to be a conclusive reason why one ought to do something (rather than something else). The problem of reason aggregation is that it is unclear whether there is such a method of aggregation and, if there is, whether it is additive.