Every motive has an intended effect on the world. When the faculty of desire is what is driving us, we first examine what possibilities the world leaves open, selecting what effect at which we wish to aim. This is not the way acting on the practical law works, though. The only possible object of the practical law is the Good, because the Good is always an appropriate object for the practical law.
We must avoid the danger of understanding the practical law as simply the law that tells us to pursue the good, and instead understand the good as simply that at which the practical law aims. If we did not understand the good in terms of the practical law, we would need some other analysis by which to understand it. The only other option is the mistaken one of understanding the Good as the pursuit of pleasure, and understanding the Evil as acting so as to produce pain for yourself.
We can also fall into believing this confused theory that the good is pleasure by confusing the idea of good versus evil with the idea of good versus bad. The good, as opposed to the bad, is really just pleasure. But the good, meaning the moral good, is not. If a morally good person suffers from a painful disease, his condition is bad (painful), but he is not thereby a bad (evil) person. If a morally bad person is punished for his misdeeds, that is bad for him (painful), but it is also just and good.
The error of all past philosophical investigators of morality is that they have striven to understand the moral in terms of the good rather than vice-versa. In doing so, they also succumb to the error of understanding morality as the pursuit of pleasure, for if one desires the good, one acts in order to satisfy that desire, that is, in order to produce pleasure. Ancient philosophers commit this error openly by seeing ethics as the subject that seeks to define the good, while modern philosophers commit it less openly by defining the right as the pursuit of whatever they see as the good, be that pleasure, obedience to God, or something else.
The moral law is equivalent to the idea of freedom, that is, causation from the noumenal to the phenomenal. The noumenal cannot be sensed, and so moral law can be grasped intellectually but its application cannot be seen. We know, rather, when something is morally right by intellectually considering whether it is coherent that that sort of action could be universally performed.
The idea that we know about what is right and wrong through abstract reflection is called "moral rationalism". It contrasts with two erroneous approaches to knowing the right. The first alternative is "moral empiricism", which takes moral good and evil to be something we can sense in the world. The second alternative is "moral mysticism", which takes sensing the moral to be a matter of sensing a supernatural property, such as whether the action is pleasing in God's eyes. Although both are mistakes and both are potentially harmful, the greater danger lies in moral empiricism. Kant equates moral empiricism with the theory that the right is the pursuit of pleasure, and so sees it as a greater temptation than moral mysticism, which also is less dangerous because it requires its adherents to try to imagine the supernatural, a tiring task that will appeal to few.
Here Kant positions himself on two philosophical issues that remain discussed to this day. The first issue is which of the morally upright person or the good states of affairs should be taken to be more fundamental to understanding ethics. The second issue is how we know which actions are morally right and which are wrong.
The position Kant holds, that moral rightness is fundamental and that moral rightness is a matter of following a rule, is called "deontology". In Kant's case, that rule is the categorical imperative, but we can imagine other possibilities, such as the Ten Commandments. Another view that takes the idea of moral goodness to be central is "virtue ethics," a view inspired by Aristotle as well as by 18th century "moral sense" theorists such as Hume and Hutchinson. The virtue ethicist takes the central idea of ethics to be not what particular actions are right or wrong, but rather the morally virtuous character as a whole. Kant's view can actually be seen as somewhere between virtue ethics and deontology, for although he takes the mark of the moral to be following the moral law, following the moral law is a matter of having the right inner motivation—acting from duty—and not simply outwardly conforming to a rule.
Opposed to taking moral goodness as being most basic to ethics is the idea of taking good states-of-affairs as basic, and seeing the right as definable in terms of their pursuit. Kant equates this with the view that the right is the pursuit of the pleasurable. If we take this to mean the pursuit of the greatest pleasure for everyone all around, this is Jeremy Bentham's utilitarianism, summed up in his formula: "The greatest good for the greatest number." Utilitarianism is part of a broader class of "consequentialist" views, views that take the morally right to be production of good consequences, whether the value of the consequences is measured solely by amount of pleasure, or instead includes other possible goods like beauty, flourishing of nature, technological, artistic, and other achievements, etc.
Kant's place in this issue is somewhat confused by a problem in interpreting him. He explains good and evil in two ways: in one, they apply specifically to the will and are synonymous with following and breaking the moral law, while in the other, they are the object of the moral law and apply to states of affairs. Both of these usages presumably contrast with good versus bad, which refer to pleasure versus pain. We can be sure that Kant takes moral rightness to be basic, but less sure about how he interprets the concept of the good.
The second issue on which Kant takes a position in this chapter is the issue of how we detect the moral. Moral empiricism holds that we just look at the action and see—in the same way that we can see the color of the shirt of the person acting. Kant equates this with utilitarianism, and with a sort of utilitarianism that seeks only to maximize one's own pleasure, not the general pleasure. Although the equation is unfair, it is true that this is one variety of moral empiricism. Moral mysticism takes morality to be a supernatural matter. Seeing what is right is a matter of "seeing," but not using the normal senses.
We might be at first confused as to why Kant's rationalism is so different from empiricism. It seems that the cognitive and sensory faculties must cooperate either to detect when an action is satisfying the categorical imperative (rationalism) or when it is producing overall greatest pleasure (empiricism). The answer turns on the idea that we really can never be sure when we have witnessed a moral act on Kant's view, for the moral rightness of the act consists of its being caused in the right way from the noumenal world, which is by definition undetectable. But now we worry how we can ever have any idea when anyone else is acting morally. We can know when we ourselves are acting morally, for Kant claims that we have a special non-sensory grasp of the moral law and through it our noumenal autonomy. This means that at least we can know of our own moral goodness, but in a way which is suspiciously like that of the "moral mystic."