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 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. 5. How does Kant's account not lead to the belief that knowledge is just a matter of personal perspective?