This morning, after three hours, I finally finished replacing the transmission in my self-propelled lawnmower. I had done it well, no parts left over, everything went back exactly the way it was supposed to. When I turned it on, everything was perfect, except that it only goes backward! I found out that the same thing had happened to some other people. It seems that the TORO company uses the exact same transmission for its front and rear wheel drive mowers, and the only difference is inside the gear box, one has the axle gear on the left side of the driveshaft, and the other has it on the right. Somehow, a few front wheel transmissions got mixed up and ended up in the wrong box. The parts look identical on the outside. You could tell the difference if you turned the shaft and the axle did not go the right way, but who would have ever thought of doing that?  I think the Human Heart can be like this, too. A person can look perfect on the outside, everything going well, and no reason to suspect that something is totally wrong. Its only when you ask it do something that it doesn't want to do, that you find out whether it is a good heart or a worthless one.