I
like to put it that to understand something is to be able to predict the most, with the least amount of information. This ability means that you grasp what is most important, what is irrelevant, and what has relatively little effect on the situation.
Suppose two men were asked whether a particular mountain pass was going to have an avalanche in the next week. The first requests information on the temperature, the slope of the mountains, the ambient noise levels, and amount of recent snowfall. From these pieces of information, he accurately predicts the avalanche. The second man requests the position and velocity of every single particle in the universe, and with this information, he accurately predicts the avalanche.
It should be obvious that one of these men understands the mountain pass, and the other does not. Likewise, saying that if you had all of someone's genes, life history, and perfectly replicated situation, you could predict their actions tells us nothing except that you do not understand human beings.