Besides a Camera, What is the most indispensable piece of equipment to photography and why?

the most indispensable thing in a camera would have to be what your taking a picture of. rather its a person or a thing or a dog. it important because with out it what is a picture you have to have the thing you want to take a picture of ready and it can be what ever. the camera i believe is one half and the other half what your doing is the actual person place or thing that you talking about. also like your model that you are doing and what that is. the most important one of all in photography is the model think thats the most importent one.