Therefore, all that Smith and Hume have successfully shown is how people do make moral decisions, not how they ought to.
By qualified, Kant means that those goods are good insofar as they presuppose or derive their goodness from something else. However, the fact that we see ourselves as often falling short of what morality demands of us indicates we have some functional concept of the moral law.
If nature's creatures are so purposed, Kant thinks their capacity to reason would certainly not serve a purpose of self-preservation or achievement of happiness, which are better served by their natural inclinations. Particularly good are the arguments he offers in the fifth section of his essay against efforts to soften Kant's motivational rigorism -- that is, against those who claim that duty is a "backup motive", or that actions that are in line with duty and an agent's own inclinations are "over-determined".
However, Kant observes that there is one end that we all share, namely our own happiness. Last, we don't hold accountable a person whose house was destroyed by a hurricane, and is reduced to penury, because the hurricane might be said to have struck by chance.
Arguments from inclination have no real moral heft. The Categorical Imperative Kant thinks that all of our actions, whether motivated by inclination or morality, must follow some law. Autonomous action in Kantian philosophy is thought as having the will to act independently and freely, or rather to be self-governing.
Finally, Kant remarks that whilst he would like to be able to explain how morality ends up motivating us, his theory is unable to do so. We must both be determining our own actions and not pursuing some desire.