Kant

Immanuel Kant (Königsberg = Kaliningrad, near the north of Poland, 1724 – 1804) was an “anti-metaphysical”: according to him we have no way to establish what is outside our mind; he calls this unknown world “noumenon” (which means “thought thing”, what is thought), as opposed to “phenomenon” (meaning “appeared thing”, and therefore perceived). Therefore Kant directs his efforts to investigate how our mind knows. Our mind knows because it frames our feelings in some patterns it possesses by constitution; these patterns are essentially space and time. Therefore space and time are not to be considered qualities possessed by objects; they are rather the coordinates with which nature has equipped our mind, so that it can have a knowledge of things. Space and time can be considered universal, however not in the sense that they are part of the whole external world, but in the sense that they are found in the minds of all people of the world, as references within which we frame every knowledge. The same is true about unities that we are able to identify in the world: every unity we understand is not to refer to objects, but to the structure of our mind, which tends to conceive it as unitary. Today we can think that, for example, we perceive a car as a unitary object, rather than a gathered set of various pieces, each on its own, because our mind tends to unify, to gather. Even the perception of ourselves as “I” (which Kant calls the “I think”) is nothing but a tool with which nature decided to equip us, to live in this world in a human way; it is not a singular, exclusive self, but a structure with which the reason of every human being is organized and which induces it to represent to itself an idea of itself as “I”.
Since our categories will never be adequate to fully know reality (or the noumenon), and yet they continually push us to go further, since we don’t know their limits, we are inevitably condemned to a continuous mistaking, which consists in the claim to know more than our mind is able to; in order to repair this error, which by nature will always accompany us, a continuous work of self-criticism is necessary, which Kant calls “transcendental dialectic”: “dialectic” indicates a reasoning work that never stops; “transcendental” indicates that this work refers to the activity of our mind, when it abuses the transcendent categories, that is, present in our mind beyond the sensible experience. The mistake we are talking about is the temptation of reason to move away from perceptions and go ahead on its own, by pure reasoning, with the pretense of advancing in knowledge on its own, by pure theories. For this reason the work of transcendental dialectic was entitled by Kant “Critique of Pure Reason”. The errors to be corrected consist essentially of three ideas. The first idea to criticize is that of the “I”: it cannot be understood or treated as a metaphysical object, since it is nothing but a subjective effect of the unity of knowing. The second idea to criticize is that of the world: it cannot be thought of as a metaphysical object; if we treat it as a metaphysical object, four problems without solution arise: 1) does the world have a beginning or does it not? That is, is it finished or infinite? 2) Is it better to take into consideration global objects (the car) or the single parts (so many mechanical pieces each on its own, that have been assembled together)? 3) Is there freedom or a mechanistic vision is more real? 4) Is there a primordial cause at the origin of everything, or not? These four questions make no sense according to Kant: they arise from the error of considering the world as a metaphysical object, distinct from our mind. The third idea that is wrong to think of metaphysically is that of God. These three ideas cease to be errors if, instead of thinking of them metaphysically, we use them as categories that can sometimes become useful, convenient to frame unitarily our knowledge of the world.
Practice, unlike reason,  in order to function well, must be guided by exactly opposite criteria: it must find something that is within us and establish it as a universal law. This something cannot be constituted by a particular element, because otherwise it would automatically become relative, that is valid only for me. Therefore, if I want to find a guide to life, a criterion of universal practice, I must necessarily disregard any content. With these reflections we are in the “Critique of Practical Reason”. But, if we remove the contents, what’s left? The form remains. The contents indicate what to do, the form how to do it. The “how to do” corresponds in Kant to the type of will, to our intention. Then, what can become universal must be a rule concerning our will, regardless of the things to do or not to do do. Kant identifies the universal, categorical imperative, in carrying forward in ourselves a kind of will that we believe would also work as a universal law, that is, a way of wanting (not a “what to want”) that we think would be good if it were adopted by everyone. Kant’s proposal, for his reference to the universal, eventually becomes another way of expressing the saying “don’t do to others what you don’t want them to do to you”. Kant then notes that a person who obeys the categorical imperative stirs in others an inevitable feeling of respect, regardless of whether these other people will accept this feeling in themselves or will prefer to hide it from others or from themselves. In this context, freedom exists according to Kant: it is not thinkable in a metaphysical sense (compare point 3 above), but it exists as a necessary criterion that allows us to ask ourselves about our own behavior; it is a criterion that does not derive from reasoning, but is present in our mind as a postulate, an intuition, a noumenon that can and must guide our practice, our moral behavior.