In Kant's essay "Answering the Question: What is Enlightenment?", Kant defined the Enlightenment as an age shaped by the Latin motto Sapere aude ("Dare to be wise"). Kant maintained that one ought to think autonomously, free of the dictates of external authority. His work reconciled many of the differences between the rationalist and empiricist traditions of the 18th century. He had a decisive impact on the Romantic and German Idealist philosophies of the 19th century. His work has also been a starting point for many 20th century philosophers.
Kant asserted that, because of the limitations of argumentation in the absence of irrefutable evidence, no one could really know whether there is a God and an afterlife or not. For the sake of morality and as a ground for reason, Kant asserted, people are justified in believing in God, even though they could never know God's presence empirically. He explained:
All the preparations of reason, therefore, in what may be called pure philosophy, are in reality directed to those three problems only [God, the soul, and freedom]. However, these three elements in themselves still hold independent, proportional, objective weight individually. Moreover, in a collective relational context; namely, to know what ought to be done: if the will is free, if there is a God, and if there is a future world. As this concerns our actions with reference to the highest aims of life, we see that the ultimate intention of nature in her wise provision was really, in the constitution of our reason, directed to moral interests only.
The sense of an enlightened approach and the critical method required that "If one cannot prove that a thing is, he may try to prove that it is not. If he fails to do neither (as often occurs), he may still ask whether it is in his interest to accept one or the other of the alternatives hypothetically, from the theoretical or the practical point of view. Hence the question no longer is as to whether perpetual peace is a real thing or not a real thing, or as to whether we may not be deceiving ourselves when we adopt the former alternative, but we must act on the supposition of its being real." The presupposition of God, soul, and freedom was then a practical concern, for "Morality, by itself, constitutes a system, but happiness does not, unless it is distributed in exact proportion to morality. This, however, is possible in an intelligible world only under a wise author and ruler. Reason compels us to admit such a ruler, together with life in such a world, which we must consider as future life, or else all moral laws are to be considered as idle dreams... ."
Kant claimed to have created a "Copernican revolution" in philosophy. This involved two interconnected foundations of his "critical philosophy":
- the epistemology of transcendental idealism and
- the moral philosophy of the autonomy of practical reason.
These teachings placed the active, rational human subject at the center of the cognitive and moral worlds. Kant argued that the rational order of the world as known by science was not just the accidental accumulation of sense perceptions.
Conceptual unification and integration is carried out by the mind through concepts or the "categories of the understanding" operating on the perceptual manifold within space and time. The latter are not concepts, but are forms of sensibility that are a priori necessary conditions for any possible experience. Thus the objective order of nature and the causal necessity that operates within it depend on the mind's processes, the product of the rule-based activity that Kant called, "synthesis." There is much discussion among Kant scholars about the correct interpretation of this train of thought.
The 'two-world' interpretation regards Kant's position as a statement of epistemological limitation, that we are not able to transcend the bounds of our own mind, meaning that we cannot access the "thing-in-itself". However, Kant also speaks of the thing in itself or transcendental object as a product of the (human) understanding as it attempts to conceive of objects in abstraction from the conditions of sensibility. Following this line of thought, some interpreters have argued that the thing in itself does not represent a separate ontological domain but simply a way of considering objects by means of the understanding alone – this is known as the two-aspect view.
The notion of the "thing in itself" was much discussed by philosophers after Kant. It was argued that because the "thing in itself" was unknowable, its existence must not be assumed. Rather than arbitrarily switching to an account that was ungrounded in anything supposed to be the "real," as did the German Idealists, another group arose to ask how our (presumably reliable) accounts of a coherent and rule-abiding universe were actually grounded. This new kind of philosophy became known as Phenomenology, and its founder was Edmund Husserl.
With regard to morality, Kant argued that the source of the good lies not in anything outside the human subject, either in nature or given by God, but rather is only the good will itself. A good will is one that acts from duty in accordance with the universal moral law that the autonomous human being freely gives itself. This law obliges one to treat humanity – understood as rational agency, and represented through oneself as well as others – as an end in itself rather than (merely) as means to other ends the individual might hold. This necessitates practical self-reflection in which we universalize our reasons.
These ideas have largely framed or influenced all subsequent philosophical discussion and analysis. The specifics of Kant's account generated immediate and lasting controversy. Nevertheless, his theses – that the mind itself necessarily makes a constitutive contribution to its knowledge, that this contribution is transcendental rather than psychological, that philosophy involves self-critical activity, that morality is rooted in human freedom, and that to act autonomously is to act according to rational moral principles – have all had a lasting effect on subsequent philosophy.
Theory of perception
Kant defines his theory of perception in his influential 1781 work the Critique of Pure Reason, which has often been cited as the most significant volume of metaphysics and epistemology in modern philosophy. Kant maintains that our understanding of the external world had its foundations not merely in experience, but in both experience and a priori concepts, thus offering a non-empiricist critique of rationalist philosophy, which is what he and others referred to as his "Copernican revolution".
Firstly, Kant distinguishes between analytic and synthetic propositions:
- Analytic proposition: a proposition whose predicate concept is contained in its subject concept; e.g., "All bachelors are unmarried," or, "All bodies take up space."
- Synthetic proposition: a proposition whose predicate concept is not contained in its subject concept; e.g., "All bachelors are happy," or, "All bodies have weight."
An analytic proposition is true by nature of the meaning of the words in the sentence — we require no further knowledge than a grasp of the language to understand this proposition (a theory put forward by Weiner in his papers on expressionism). On the other hand, a synthetic statement is one that tells us something about the world. The truth or falsehood of synthetic statements derives from something outside their linguistic content. In this instance, weight is not a necessary predicate of the body; until we are told the heaviness of the body we do not know that it has weight. In this case, experience of the body is required before its heaviness becomes clear. Before Kant's first Critique, empiricists (cf. Hume) and rationalists (cf. Leibniz) assumed that all synthetic statements required experience to be known.
Kant, however, contests this: he claims that elementary mathematics, like arithmetic, is synthetic a priori, in that its statements provide new knowledge, but knowledge that is not derived from experience. This becomes part of his over-all argument for transcendental idealism. That is, he argues that the possibility of experience depends on certain necessary conditions — which he calls a priori forms — and that these conditions structure and hold true of the world of experience. In so doing, his main claims in the "Transcendental Aesthetic" are that mathematic judgments are synthetic a priori and in addition, that Space and Time are not derived from experience but rather are its preconditions.
Once we have grasped the functions of basic arithmetic, we do not need any empirical experience to know that 100 + 100 = 200, and so it appears that arithmetic is analytic. However, that it is analytic can be disproved by considering the calculation 5 + 7 = 12: there is nothing in the numbers 5 and 7 by which the number 12 can be inferred. Thus "5 + 7" and "the cube root of 1,728" or "12" are not analytic because their reference is the same but their sense is not — the mathematical judgment "5 + 7 = 12" tells us something new about the world. It is self-evident, and undeniably a priori, but at the same time it is synthetic. Thus Kant proved that a proposition can be synthetic and known a priori.
Kant asserts that experience is based both on the perception of external objects and a priori knowledge. The external world, he writes, provides those things that we sense. But it is our mind that processes this information and gives it order, allowing us to comprehend it. Our mind supplies the conditions of space and time to experience objects. According to the "transcendental unity of apperception", the concepts of the mind (Understanding) and the perceptions or intuitions that garner information from phenomena (Sensibility) are synthesized by comprehension. Without the concepts, perceptions are nondescript; without the perceptions, concepts are meaningless — thus the famous statement, "Thoughts without content are empty, intuitions (perceptions) without concepts are blind."
Kant also claims that an external environment is necessary for the establishment of the self. Although Kant would want to argue that there is no empirical way of observing the self, we can see the logical necessity of the self when we observe that we can have different perceptions of the external environment over time. By uniting all of these general representations into one global representation, we can see how a transcendental self emerges. "I am therefore conscious of the identical self in regard to the manifold of the representations that are given to me in an intuition because I call them all together my representations."
Categories of the Faculty of Understanding
Kant deemed it obvious that we have some objective knowledge of the world, such as, say, Newtonian physics. But this knowledge relies on synthetic, a priori laws of nature, like causality and substance. The problem, then, is how this is possible. Kant's solution was to reason that the subject must supply laws that make experience of objects possible, and that these laws are the synthetic, a priori laws of nature that we know apply to all objects before we experience them. So, to deduce all these laws, Kant examined experience in general, dissecting in it what is supplied by the mind from what is supplied by the given intuitions. What has just been explicated is commonly called a transcendental reduction.
To begin with, Kant's distinction between the a posteriori being contingent and particular knowledge, and the a priori being universal and necessary knowledge, must be kept in mind. For if we merely connect two intuitions together in a perceiving subject, the knowledge is always subjective because it is derived a posteriori, when what is desired is for the knowledge to be objective, that is, for the two intuitions to refer to the object and hold good of it necessarily universally for anyone at anytime, not just the perceiving subject in its current condition. What else is equivalent to objective knowledge besides the a priori, that is to say, universal and necessary knowledge? Nothing else, and hence before knowledge can be objective, it must be incorporated under an a priori category of the understanding.
For example, say a subject says, "The sun shines on the stone; the stone grows warm," which is all he perceives in perception. His judgment is contingent and holds no necessity. But if he says, "The sunshine causes the stone to warm," he subsumes the perception under the category of causality, which is not found in the perception, and necessarily synthesizes the concept sunshine with the concept heat, producing a necessarily universally true judgment.
To explain the categories in more detail, they are the preconditions of the construction of objects in the mind. Indeed, to even think of the sun and stone presupposes the category of subsistence, that is, substance. For the categories synthesize the random data of the sensory manifold into intelligible objects. This means that the categories are also the most abstract things one can say of any object whatsoever, and hence one can have an a priori cognition of the totality of all objects of experience if one can list all of them. To do so, Kant formulates another transcendental deduction.
Judgments are, for Kant, the preconditions of any thought. Man thinks via judgments, so all possible judgments must be listed and the perceptions connected within them put aside, so as to make it possible to examine the moments when the understanding is engaged in constructing judgments. For the categories are equivalent to these moments, in that they are concepts of intuitions in general, so far as they are determined by these moments universally and necessarily. Thus by listing all the moments, one can deduce from them all of the categories.
One may now ask: How many possible judgments are there? Kant believed that all the possible propositions within Aristotle's syllogistic logic are equivalent to all possible judgments, and that all the logical operators within the propositions are equivalent to the moments of the understanding within judgments. Thus he listed Aristotle's system in four groups of three: quantity (universal, particular, singular), quality (affirmative, negative, infinite), relation (categorical, hypothetical, disjunctive) and modality (problematic, assertoric, apodeictic). The parallelism with Kant's categories is obvious: quantity (unity, plurality, totality), quality (reality, negation, limitation), relation (substance, cause, community) and modality (possibility, existence, necessity).
The fundamental building blocks of experience, i.e. objective knowledge, are now in place. First there is the sensibility, which supplies the mind with intuitions, and then there is the understanding, which produces judgments of these intuitions and can subsume them under categories. These categories lift the intuitions up out of the subject's current state of consciousness and place them within consciousness in general, producing universally necessary knowledge. For the categories are innate in any rational being, so any intuition thought within a category in one mind is necessarily subsumed and understood identically in any mind. In other words we filter what we see and hear.
Transcendental schema doctrine
Kant ran into a problem with his theory that the mind plays a part in producing objective knowledge. Intuitions and categories are entirely disparate, so how can they interact? Kant's solution is the (transcendental) schema: a priori principles by which the transcendental imagination connects concepts with intuitions through time. All the principles are temporally bound, for if a concept is purely a priori, as the categories are, then they must apply for all times. Hence there are principles such as substance is that which endures through time, and the cause must always be prior to the effect.
Kant developed his moral philosophy in three works: Groundwork of the Metaphysic of Morals (1785), Critique of Practical Reason (1788), and Metaphysics of Morals (1797).
In the Groundwork, Kant's method involves trying to convert our everyday, obvious, rational knowledge of morality into philosophical knowledge. The latter two works followed a method of using "practical reason", which is based only on things about which reason can tell us, and not deriving any principles from experience, to reach conclusions which can be applied to the world of experience (in the second part of The Metaphysic of Morals).
Kant is known for his theory that there is a single moral obligation, which he called the "Categorical Imperative", and is derived from the concept of duty. Kant defines the demands of the moral law as "categorical imperatives". Categorical imperatives are principles that are intrinsically valid; they are good in and of themselves; they must be obeyed by everyone in all situations and circumstances, if our behavior is to observe the moral law. The Categorical Imperative generates all other moral obligations, and they can be tested against it. Kant also stated that the moral means and ends can be applied to the categorical imperative, that rational beings can pursue certain "ends" using the appropriate "means". Ends based on physical needs or wants can create merely hypothetical imperatives. The categorical imperative can only be based on something that is an "end in itself", that is, an end that is not a means to some other need, desire, or purpose. Kant believed that the moral law is a principle of reason itself, and is not based on contingent facts about the world, such as what would make us happy, but to act on the moral law which has no other motive than "worthiness of being happy". Accordingly, he believed that moral obligation applies only to rational agents.
Unlike a hypothetical imperative, a categorical imperative is an unconditional obligation; that is, it has the force of an obligation regardless of our will or desires In Groundwork of the Metaphysic of Morals (1785) Kant enumerated three formulations of the categorical imperative that he believed to be roughly equivalent.
Kant believed that if an action is not done with the motive of duty, then it is without moral value. He thought that every action should have pure intention behind it; otherwise it was meaningless. The final result was not the most important aspect of an action, but how the person felt while carrying out the action was the time at which value was set to the result.
In Groundwork of the Metaphysic of Morals, Kant also posited the "counter-utilitarian idea that there is a difference between preferences and values, and that considerations of individual rights temper calculations of aggregate utility", a concept that is an axiom in economics:
Everything has either a price or a dignity. Whatever has a price can be replaced by something else as its equivalent; on the other hand, whatever is above all price, and therefore admits of no equivalent, has a dignity. But that which constitutes the condition under which alone something can be an end in itself does not have mere relative worth, i.e., price, but an intrinsic worth, i.e., a dignity. (p. 53, italics in original).
A phrase quoted by Kant, which is used to summarize the counter-utilitarian nature of his moral philosophy, is Fiat justitia, pereat mundus, ("Let justice be done, though the world perish"), which he translates loosely as "Let justice reign even if all the rascals in the world should perish from it". This appears in his 1795 Perpetual Peace (Zum ewigen Frieden. Ein philosophischer Entwurf.), Appendix 1.
The first formulation (Formula of Universal Law) of the moral imperative "requires that the maxims be chosen as though they should hold as universal laws of nature" . This formulation in principle has as its supreme law the creed "Always act according to that maxim whose universality as a law you can at the same time will" and is the "only condition under which a will can never come into conflict with itself [....]"
One interpretation of the first formulation is called the "universalizability test". An agent's maxim, according to Kant, is his "subjective principle of human actions": that is, what the agent believes is his reason to act. The universalisability test has five steps:
- Find the agent's maxim (i.e., an action paired with its motivation). Take for example the declaration "I will lie for personal benefit". Lying is the action; the motivation is to fulfill some sort of desire. Paired together, they form the maxim.
- Imagine a possible world in which everyone in a similar position to the real-world agent followed that maxim. With no exception of one's self. This is in order for you to hold people to the same principle required of yourself.
- Decide whether any contradictions or irrationalities arise in the possible world as a result of following the maxim.
- If a contradiction or irrationality arises, acting on that maxim is not allowed in the real world.
- If there is no contradiction, then acting on that maxim is permissible, and is sometimes required.
(For a modern parallel, see John Rawls' hypothetical situation, the original position.)
The second formulation (or Formula of the End in Itself) holds that "the rational being, as by its nature an end and thus as an end in itself, must serve in every maxim as the condition restricting all merely relative and arbitrary ends". The principle dictates that you "[a]ct with reference to every rational being (whether yourself or another) so that it is an end in itself in your maxim", meaning that the rational being is "the basis of all maxims of action" and "must be treated never as a mere means but as the supreme limiting condition in the use of all means, i.e., as an end at the same time".
The third formulation (Formula of Autonomy) is a synthesis of the first two and is the basis for the "complete determination of all maxims". It says "that all maxims which stem from autonomous legislation ought to harmonize with a possible realm of ends as with a realm of nature". In principle, "So act as if your maxims should serve at the same time as the universal law (of all rational beings)", meaning that we should so act that we may think of ourselves as "a member in the universal realm of ends", legislating universal laws through our maxims (that is, a code of conduct), in a "possible realm of ends". None may elevate themselves above the universal law, therefore it is one's duty to follow the maxim(s).
Religion Within the Limits of Reason
Kant articulates his strongest criticisms of the organization and practices of religious organizations to those that encourage what he sees as a religion of counterfeit service to God. Among the major targets of his criticism are external ritual, superstition and a hierarchical church order. He sees all of these as efforts to make oneself pleasing to God in ways other than conscientious adherence to the principle of moral rightness in choosing one's actions. The severity of Kant's criticisms on these matters, along with his rejection of the possibility of theoretical proofs for the existence of God and his philosophical re-interpretation of some basic Christian doctrines, allow interpretations that see Kant as thoroughly hostile to religion in general and Christianity in particular (e.g., Walsh 1967). Nevertheless, other interpreters consider that Kant was trying to mark off a defensible rational core of Christian belief. Kant sees in Jesus Christ the affirmation of a "pure moral disposition of the heart" that "can make man well-pleasing to God".
Idea of freedom
In the Critique of Pure Reason, Kant distinguishes between the transcendental idea of freedom, which as a psychological concept is "mainly empirical" and refers to "the question whether we must admit a power of spontaneously beginning a series of successive things or states" as a real ground of necessity in regard to causality, and the practical concept of freedom as the independence of our will from the "coercion" or "necessitation through sensuous impulses". Kant finds it a source of difficulty that the practical idea of freedom is founded on the transcendental idea of freedom, but for the sake of practical interests uses the practical meaning, taking "no account of... its transcendental meaning," which he feels was properly "disposed of" in the Third Antinomy, and as an element in the question of the freedom of the will is for philosophy "a real stumbling-block" that has "embarrassed speculative reason".
Kant calls practical "everything that is possible through freedom", and the pure practical laws that are never given through sensuous conditions but are held analogously with the universal law of causality are moral laws. Reason can give us only the "pragmatic laws of free action through the senses", but pure practical laws given by reason a priori dictate "what ought to be done".
Categories of freedom
In the Critique of Practical Reason, at the end of the second Main Part of the Analytics, Kant introduces the categories of freedom, in analogy with the categories of understanding their practical counterparts. Kant's categories of freedom apparently function primarily as conditions for the possibility for actions (i) to be free, (ii) to be understood as free and (iii) to be morally evaluated. For Kant, although actions as theoretical objects are constituted by means of the theoretical categories, actions as practical objects (objects of practical use of reason, and which can be good or bad) are constituted by means of the categories of freedom. Only in this way can actions, as phenomena, be a consequence of freedom, and be understood and evaluated as such.
Kant discusses the subjective nature of aesthetic qualities and experiences in Observations on the Feeling of the Beautiful and Sublime, (1764). Kant's contribution to aesthetic theory is developed in the Critique of Judgment (1790) where he investigates the possibility and logical status of "judgments of taste." In the "Critique of Aesthetic Judgment," the first major division of the Critique of Judgment, Kant used the term "aesthetic" in a manner that, according to Kant scholar W.H. Walsh, differs from its modern sense. Prior to this, in the Critique of Pure Reason, to note essential differences between judgments of taste, moral judgments, and scientific judgments, Kant abandoned the term "aesthetic" as "designating the critique of taste," noting that judgments of taste could never be "directed" by "laws a priori". After A. G. Baumgarten, who wrote Aesthetica (1750–58), Kant was one of the first philosophers to develop and integrate aesthetic theory into a unified and comprehensive philosophical system, utilizing ideas that played an integral role throughout his philosophy.
In the chapter "Analytic of the Beautiful" of the Critique of Judgment, Kant states that beauty is not a property of an artwork or natural phenomenon, but is instead a consciousness of the pleasure that attends the 'free play' of the imagination and the understanding. Even though it appears that we are using reason to decide what is beautiful, the judgment is not a cognitive judgment, "and is consequently not logical, but aesthetical" (§ 1). A pure judgement of taste is in fact subjective insofar as it refers to the emotional response of the subject and is based upon nothing but esteem for an object itself: it is a disinterested pleasure, and we feel that pure judgements of taste, i.e. judgements of beauty, lay claim to universal validity (§§20–22). It is important to note that this universal validity is not derived from a determinate concept of beauty but from common sense (§40). Kant also believed that a judgement of taste shares characteristics engaged in a moral judgement: both are disinterested, and we hold them to be universal. In the chapter "Analytic of the Sublime" Kant identifies the sublime as an aesthetic quality that, like beauty, is subjective, but unlike beauty refers to an indeterminate relationship between the faculties of the imagination and of reason, and shares the character of moral judgments in the use of reason. The feeling of the sublime, itself officially divided into two distinct modes (the mathematical and the dynamical sublime), describes two subjective moments, both of which concern the relationship of the faculty of the imagination to reason. Some commentators, however, argue that Kant's critical philosophy contains a third kind of the sublime, the moral sublime, which is the aesthetic response to the moral law or a representation thereof, and a development of the "noble" sublime in Kant's theory of 1764. The mathematical sublime is situated in the failure of the imagination to comprehend natural objects that appear boundless and formless, or appear "absolutely great" (§ 23–25). This imaginative failure is then recuperated through the pleasure taken in reason's assertion of the concept of infinity. In this move the faculty of reason proves itself superior to our fallible sensible self (§§ 25–26). In the dynamical sublime there is the sense of annihilation of the sensible self as the imagination tries to comprehend a vast might. This power of nature threatens us but through the resistance of reason to such sensible annihilation, the subject feels a pleasure and a sense of the human moral vocation. This appreciation of moral feeling through exposure to the sublime helps to develop moral character.
Kant had developed the distinction between an object of art as a material value subject to the conventions of society and the transcendental condition of the judgment of taste as a "refined" value in the propositions of his Idea of A Universal History (1784). In the Fourth and Fifth Theses of that work he identified all art as the "fruits of unsociableness" due to men's "antagonism in society", and in the Seventh Thesis asserted that while such material property is indicative of a civilized state, only the ideal of morality and the universalization of refined value through the improvement of the mind of man "belongs to culture".
In Perpetual Peace: A Philosophical Sketch, Kant listed several conditions that he thought necessary for ending wars and creating a lasting peace. They included a world of constitutional republics. His classical republican theory was extended in the Science of Right, the first part of the Metaphysics of Morals (1797).
Kant's political thought can be summarized as republican government and international organization. "In more characteristically Kantian terms, it is doctrine of the state based upon the law (Rechtsstaat) and of eternal peace. Indeed, in each of these formulations, both terms express the same idea: that of legal constitution or of 'peace through law'. Taken simply by itself, Kant's political philosophy, being essentially a legal doctrine, rejects by definition the opposition between moral education and the play of passions as alternate foundations for social life. The state is defined as the union of men under law. The state rightly so called is constituted by laws which are necessary a priori because they flow from the very concept of law. A regime can be judged by no other criteria nor be assigned any other functions, than those proper to the lawful order as such." 
He opposed "democracy," which at his time meant direct democracy, believing that majority rule posed a threat to individual liberty. He stated, "...democracy is, properly speaking, necessarily a despotism, because it establishes an executive power in which 'all' decide for or even against one who does not agree; that is, 'all,' who are not quite all, decide, and this is a contradiction of the general will with itself and with freedom." As with most writers at the time, he distinguished three forms of government i.e. democracy, aristocracy, and monarchy with mixed government as the most ideal form of it.