The psychology of perception suggests the existence of a common cognitive system that treats all or most sensorily conveyed meanings in the same way.[citation needed] If all signs must also be objects of perception, there is every reason to believe that their modality will determine at least part of their nature. Thus, the sensory modalities will be visual, auditory, tactile, olfactory, gustatory, kinesthetic, etc. A list of sign types would include: writing, symbol, index, image, map, graph, diagram, etc.