Explorer won't start, part II

First, thank you very much for your help. Here's my update. I tested for a spark and it is there. I listened to the fuel pump and heard it humming. My plan now is to replace the fuel filter because the car has 195k miles and is probably on its original filter anyway. I'll replace the air filter while I'm at it. If that doesn't clear up the problem, I'm going to take it in and have the fuel pressure tested to see if maybe the pump has gone bad. Is there anything else that could be causing the problem. Engines only need three things, as far as I know: fuel, spark, and air. Is it possible that the computer could be sensing an error and preventing the car from starting? Thanks again.