As I understand it, there is a finite number of photo receptors in our eyes, laid out in an irregular mosaic. When light enters our eyes and strikes the retina, the photo receptors send an electrical signal along a chain of nerve cells to the brain. The Brain knows the signal came from a photo receptor because it knows where each and every nerve it wired to. Kind of like a security guard sitting at a control panel, if an alarm is tripped a light comes on, the guard knows where the intruder is because the light is labelled. From there the brain cobbles the signals together into our perception.