We can answer the question of how one uses oxidation numbers to decide whether or not a reaction is a reduction (or whether or not it is an oxidation) separately from the issue of assigning oxidation numbers in a particular reaction. For example in photosynthesis one starts with carbon dioxide (in which each carbon has an oxidation number of +4) and converts it to a hexose (C6H12O6, in which the average oxidation number of each carbon atom is 0. Would you say that a carbon atom is getting oxidized, reduced or neither?