Weather is caused by a bunch of molecules. Since we don’t know exactly how the molecules will behave, weather forecasters make lots of models and then look at the average. That’s why they say things like “There’s a 70% chance of rain”—perhaps they’ve made 100 models, and it rained in 70 of them. (As described by Nate Silver in The Signal and the Noise.)
If we knew exactly what each molecule was, though, we could predict the weather with perfect accuracy.
Now, consider the extent to which the course of your life is determined by prior causes.