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

the most indispensable piece of equipment to photography is A GOOD QUALITY CIRCULAR POLARIZING FILTER because in photography started with landscape photography, and in this endeavor. A WIDE APERTURE LENS because A high quality zoom lens with a wide aperture can set you back thousands of dollars. PHOTOSHOP LIGHT ROOM because maybe this doesn’t qualify as “equipment,” but if you haven’t taken the dive to learn Light room. A STROBE because there are very good third party alternatives out there for much less cost. A TRIPOD because photos were sharp enough for my purposes.