I have lived all my life observing the laws of Physics as they apply themselves to everything. Because of these laws I knew that if I didn't open the door before trying to walk through it, I would bruise more than my ego! There is nothing else needed. There is no need to affirm or deny the existence of anything except the laws of Physics. It's only when someone comes around and says "An all powerful, all knowing god controls everything! All those who deny god's existence are atheists!"
I don't need to deny the existence of god, reality does it for me!