Questions on Kant's Epistemology

1. What does it mean to say that, for Hume, our knowledge of cause and effect is not a priori but only a posteriori, a matter of habit?
2. Why is this a threat to science?
3. How does Kant's account of the categories of the mind deal with that threat?
4. What does it mean to say that "synthetic a priori" knowledge depends on these "transcendental" categories?
5. Why does Kant's account not lead to the belief that knowledge is just a matter of personal perspective? (See especially pp. 319-20)