Kant’s three major volumes are entitled critiques, and his entire philosophy focuses on applying his critical method to philosophical problems. The correct method in philosophy, according to Kant, is not to speculate on the nature of the world around us but to perform a critique of our mental faculties, investigating what we can know, defining the limits of knowledge, and determining how the mental processes by which we make sense of the world affect what we know. This change in method represents what Kant calls a Copernican revolution in philosophy. Just as Copernicus turned astronomy on its head in the sixteenth century by arguing that the sun, not the earth, is the center of the solar system, Kant turns philosophy on its head by arguing that we will find the answers to our philosophical problems in an examination of our mental faculties rather than in metaphysical speculation about the universe around us. One part of this revolution is the suggestion that the mind is not a passive receptor but that it actively shapes our perception of reality. Another is a general shift, which remains to this day, from metaphysics toward epistemology. That is, the question of what reality actually consists of has become less central than the question of what we can know about reality and how we can know it.
Kant’s emphasis on the role our mental faculties play in shaping our experience implies a sharp distinction between phenomena and noumena. Noumena are “things-in-themselves,” the reality that exists independent of our mind, whereas phenomena are appearances, reality as our mind makes sense of it. According to Kant, we can never know with certainty what is “out there.” Since all our knowledge of the external world is filtered through our mental faculties, we can know only the world that our mind presents to us. That is, all our knowledge is only knowledge of phenomena, and we must accept that noumena are fundamentally unknowable. Idealism is the name given to the various strands of philosophy that claim the world is made up primarily of mental ideas, not of physical things. Kant differs from many idealists in that he does not deny the existence of an external reality and does not even think that ideas are more fundamental than things. However, he argues that we can never transcend the limitations and the contextualization provided by our minds, so that the only reality we will ever know is the reality of phenomena.
Kant inherits from Hume the problem of how we can infer necessary and universal truths from experience when all experience is by its nature contingent and particular. We actually experience individual sights and sounds and so on. We cannot “experience” a physical law or a relation of cause and effect. So if we cannot see, smell, or hear causation, how can we infer that some events cause others? Kant phrases this question more generally as the question of how synthetic a priori knowledge is possible. That is, how can we know things that are necessary and universal but not self-evident or definitional? Kant’s ingenious solution is that synthetic a priori knowledge is possible because our mental faculties organize experience according to certain categories so that these categories become necessary and universal features of our experience. For instance, we do not find causation in nature so much as we cannot not find causation in nature. It is a feature of the way our minds make sense of reality that we perceive causes and effects everywhere at work. For Kant, then, the category of the synthetic a priori is the key to explaining how we gain substantive knowledge about the world.
Ethical theorists can be roughly divided into two camps: those who consider an action moral or immoral depending on the motive behind it and those who consider an action moral or immoral depending on the consequences it produces. Kant is firmly in the former camp, making him a deontologist rather than a consequentialist when it comes to ethics. (The word deontology derives from the Greek roots deon, “duty,” and logos, “science.”) Kant argues that we are subject to moral judgment because we are able to deliberate and give reasons for our actions, so moral judgment should be directed at our reasons for acting. While we can and should take some care to ensure that our actions produce good consequences, the consequences of our actions are not themselves subject to our reason, so our reason is not fully responsible for the consequences of the actions it endorses. Reason can only be held responsible for endorsing certain actions, and so it is only the actions, and the motives behind them, that are open to moral judgment.
Every theory of ethics must give an answer to the question “Or else what?” That is, we must be able to explain why good is good and bad is bad. Christians answer the “Or else what?” question with the threat of eternal damnation, while Utilitarians answer that, since happiness is the greatest good, bad actions produce unhappiness, and unhappiness is bad in and of itself. Kant, by contrast, argues that since reason is the source of morality, goodness and badness should be dictated by reason. To act badly, according to Kant, is to violate the maxims laid out by one’s reason, or to formulate maxims that one could not consistently will as universal laws. In other words, immorality is a form of irrationality: badness results from violating the laws of reason. According to Kant, our rationality is what makes us human, so by acting irrationally, and hence immorally, we also compromise our humanity. Kant’s answer to the question “Or else what?” is that we diminish ourselves as rational human beings by acting immorally. Only by behaving rationally do we show ourselves to be autonomous beings, in control of the passions and appetites that might lead us to act against our better judgment.