It is precisely in knowing its limits that philosophy consists.

Morality is thus the relation of actions to the autonomy of the will, that is, to a possible giving of universal law through its maxims. An action that can coexist with the autonomy of the will is permitted; one that does not accord with it is forbidden. A will whose maxims necessarily harmonize with the laws of autonomy is a holy, absolutely good will. The dependence upon the principle of autonomy of a will that is not absolutely good (moral necessitation) is obligation. This, accordingly, cannot be attributed to a holy being. The objective of an action from obligation is called duty.

... happiness is not an ideal of reason, but of imagination, resting on merely empirical grounds,

I maintain that in every special natural doctrine only so much science proper is to be met with as mathematics; for... science proper, especially of nature, requires a pure portion, lying at the foundation of the empirical, and based upon à priori knowledge of natural things. ...the conception should be constructed. But the cognition of the reason through construction of conceptions is mathematical. A pure philosophy of nature in general, namely, one that only investigates what constitutes a nature in general, may thus be possible without mathematics; but a pure doctrine of nature respecting determinate natural things (corporeal doctrine and mental doctrine), is only possible by means of mathematics; and as in every natural doctrine only so much science proper is to be met with therein as there is cognition à priori, a doctrine of nature can only contain so much science proper as there is in it of applied mathematics.

All natural philosophers, who wished to proceed mathematically in their work, have hence invariably (although unknown to themselves) made use of metaphysical principles, and must make use of such, it matters not how energetically they may otherwise repudiate any claim of metaphysics on their science.

All true metaphysics is taken from the essential nature of the thinking faculty itself, and therefore in nowise invented, since it is not borrowed from experience, but contains the pure operations of thought, that is, conceptions and principles à priori, which the manifold of empirical presentations first of all brings into legitimate connection, by which it can become empirical knowledge, i.e. experience. ...mathematical physicists were thus quite unable to dispense with such metaphysical principles...

Natural science is throughout either a pure or an applied doctrine of motion.

I have in this treatise followed the mathematical method, if not with all strictness, at least imitatively, not in order, by a display of profundity, to procure a better reception for it, but because I believe such a system to be quite capable of it, and that perfection may in time be obtained by a cleverer hand, if stimulated by this sketch, mathematical investigators of nature should find it not unimportant to treat the metaphysical portion, which anyway cannot be got rid of, as a special fundamental department of general physics, and to bring it into unison with the mathematical doctrine of motion.

Newton... (after having remarked that geometry only requires two of the mechanical actions which it postulates, namely, to describe a straight line and a circle) says: geometry is proud of being able to achieve so much while taking so little from extraneous sources. One might say of metaphysics, on the other hand: it stands astonished, that with so much offered it by pure mathematics it can effect so little. In the meantime, this little is something which mathematics indispensably requires in its application to natural science, which, inasmuch as it must here necessarily borrow from metaphysics, need not be ashamed to allow itself to be seen in company with the latter.

Even if there never have been actions arising from such pure sources, what is at issue here is not whether this or that happened; that, instead, reason by itself and independently of all appearances commands what ought to happen; that, accordingly, actions of which the world has perhaps so far given no example, and whose very practicability might be very much doubted by one who bases everything on experience, are still inflexibly commanded by reason … because … duty … lies, prior to all experience, in the idea of a reason determing the will by means of apriori grounds.

I do not, therefore, need any penetrating acuteness to see what I have to do in order that my volition be morally good. Inexperienced in the course of the world, incapable of being prepared for whatever might come to pass in it, I ask myself only: can you also will that your maxim become a universal law?

What is the use of working toward a lawful civic constitution among individuals, i.e., toward the creation of a commonwealth? The same unsociability which drives man to this causes any single commonwealth to stand in unrestricted freedom in relation to others; consequently, each of them must expect from another precisely the evil which oppressed the individuals and forced them to enter into a lawful civic state. The friction among men, the inevitable antagonism, which is a mark of even the largest societies and political bodies, is used by Nature as a means to establish a condition of quiet and security. Through war, through the taxing and never-ending accumulation of armament, through the want which any state, even in peacetime, must suffer internally, Nature forces them to make at first inadequate and tentative attempts; finally, after devastations, revolutions, and even complete exhaustion, she brings them to that which reason could have told them at the beginning and with far less sad experience, to wit, to step from the lawless condition of savages into a league of nations. In a league of nations, even the smallest state could expect security and justice, not from its own power and by its own decrees, but only from this great league of nations … from a united power acting according to decisions reached under the laws of their united will.

All wars are accordingly so many attempts (not in the intention of man, but in the intention of Nature) to establish new relations among states, and through the destruction or at least the dismemberment of all of them to create new political bodies, which, again, either internally or externally, cannot maintain themselves and which must thus suffer like revolutions; until finally, through the best possible civic constitution and common agreement and legislation in external affairs, a state is created which, like a civic commonwealth, can maintain itself automatically.

Is it reasonable to assume a purposiveness in all the parts of nature and to deny it to the whole?

To a high degree we are, through art and science, cultured. We are civilized — perhaps too much for our own good — in all sorts of social grace and decorum. But to consider ourselves as having reached morality — for that, much is lacking. The ideal of morality belongs to culture; its use for some simulacrum of morality in the love of honor and outward decorum constitutes mere civilization. So long as states waste their forces in vain and violent self-expansion, and thereby constantly thwart the slow efforts to improve the minds of their citizens by even withdrawing all support from them, nothing in the way of a moral order is to be expected. For such an end, a long internal working of each political body toward the education of its citizens is required. Everything good that is not based on a morally good disposition, however, is nothing but pretense and glittering misery. In such a condition the human species will no doubt remain until, in the way I have described, it works its way out of the chaotic conditions of its international relations.

The history of mankind can be seen, in the large, as the realization of Nature’s secret plan to bring forth a perfectly constituted state as the only condition in which the capacities of mankind can be fully developed, and also bring forth that external relation among states which is perfectly adequate to this end.

Everyone can see that philosophy can have her belief in a millennium, but her millennarianism is not Utopian, since the Idea can help, though only from afar, to bring the millennium to pass. The only question is: Does Nature reveal anything of a path to this end? And I say: She reveals something, but very little. This great revolution seems to require so long for its completion that the short period during which humanity has been following this course permits us to determine its path and the relation of the parts to the whole with as little certainty as we can determine, from all previous astronomical observation, the path of the sun and his host of satellites among the fixed stars. Yet, on the fundamental premise of the systematic structure of the cosmos and from the little that has been observed, we can confidently infer the reality of such a revolution. Moreover, human nature is so constituted that we cannot be indifferent to the most remote epoch our race may come to, if only we may expect it with certainty. Such indifference is even less possible for us, since it seems that our own intelligent action may hasten this happy time for our posterity. For that reason, even faint indications of approach to it are very important to us.

A philosophical attempt to work out a universal history according to a natural plan directed to achieving the civic union of the human race must be regarded as possible and, indeed, as contributing to this end of Nature.

A metaphysics of morals is therefore indispensably necessary, not merely because of a motive to speculation — for investigating the source of the practical basic principles that lie a priori in our reason — but also because morals themselves remain subject to all sorts of corruption as long as we are without that clue and supreme norm by which to appraise them correctly...

Human reason has this peculiar fate that in one species of its knowledge it is burdened by questions which, as prescribed by the very nature of reason itself, it is not able to ignore, but which, as transcending all its powers, it is also not able to answer.

Mathematics, from the earliest times to which the history of human reason can reach, has followed, among that wonderful people of the Greeks, the safe way of science. But it must not be supposed that it was as easy for mathematics as for logic, in which reason is concerned with itself alone, to find, or rather to make for itself that royal road. I believe, on the contrary, that there was a long period of tentative work (chiefly still among the Egyptians), and that the change is to be ascribed to a revolution, produced by the happy thought of a single man, whose experiments pointed unmistakably to the path that had to be followed, and opened and traced out for the most distant times the safe way of a science. The history of that intellectual revolution, which was far more important than the passage round the celebrated Cape of Good Hope, and the name of its fortunate author, have not been preserved to us. ... A new light flashed on the first man who demonstrated the properties of the isosceles triangle (whether his name was Thales or any other name), for he found that he had not to investigate what he saw hi the figure, or the mere concepts of that figure, and thus to learn its properties; but that he had to produce (by construction) what he had himself, according to concepts a priori, placed into that figure and represented in it, so that, in order to know anything with certainty a priori, he must not attribute to that figure anything beyond what necessarily follows from what he has himself placed into it, in accordance with the concept.

All our knowledge falls with the bounds of experience.

It is therefore correct to say that the senses do not err — not because they always judge rightly, but because they do not judge at all.

A plant, an animal, the regular order of nature — probably also the disposition of the whole universe — give manifest evidence that they are possible only by means of and according to ideas; that, indeed, no one creature, under the individual conditions of its existence, perfectly harmonizes with the idea of the most perfect of its kind — just as little as man with the idea of humanity, which nevertheless he bears in his soul as the archetypal standard of his actions; that, notwithstanding, these ideas are in the highest sense individually, unchangeably, and completely determined, and are the original causes of things; and that the totality of connected objects in the universe is alone fully adequate to that idea.

Metaphysics has as the proper object of its enquiries three ideas only: God, freedom, and immortality.

Human reason is by nature architectonic.

This proof can at most, therefore, demonstrate the existence of an architect of the world, whose efforts are limited by the capabilities of the material with which he works, but not of a creator of the world, to whom all things are subject.

All human knowledge begins with intuitions, proceeds from thence to concepts, and ends with ideas.

Philosophical knowledge is the knowledge gained by reason from concepts; mathematical knowledge is the knowledge gained by reason from the construction of concepts.

I have no knowledge of myself as I am, but merely as I appear to myself.

Psychologists have hitherto failed to realize that imagination is a necessary ingredient of perception itself.

Abbot Terrasson tells us that if the size of a book were measured not by the number of its pages but by the time required to understand it, then we could say about many books that they would be much shorter were they not so short.

When Galilei let balls of a particular weight, which he had determined himself, roll down an inclined plain, or Torricelli made the air carry a weight, which he had previously determined to be equal to that of a definite volume of water; or when, in later times, Stahl changed metal into lime, and lime again into metals, by withdrawing and restoring something, a new light flashed on all students of nature. They comprehended that reason has insight into that only, which she herself produces on her own plan, and that she must move forward with the principles of her judgments, according to fixed law, and compel nature to answer her questions, but not let herself be led by nature, as it were in leading strings, because otherwise accidental observations made on no previously fixed plan, will never converge towards a necessary law, which is the only thing that reason seeks and requires. Reason, holding in one hand its principles, according to which concordant phenomena alone can be admitted as laws of nature, and in the other hand the experiment, which it has devised according to those principles, must approach nature, in order to be taught by it: but not in the character of a pupil, who agrees to everything the master likes, but as an appointed judge, who compels the witnesses to answer the questions which he himself proposes. Therefore even the science of physics entirely owes the beneficial revolution in its character to the happy thought, that we ought to seek in nature (and not import into it by means of fiction) whatever reason must learn from nature, and could not know by itself, and that we must do this in accordance with what reason itself has originally placed into nature. Thus only has the study of nature entered on the secure method of a science, after having for many centuries done nothing but grope in the dark.

I have therefore found it necessary to deny knowledge in order to make room for faith.

Criticism alone can sever the root of materialism, fatalism, atheism, free-thinking, fanaticism, and superstition, which can be injurious universally; as well as of idealism and skepticism, which are dangerous chiefly to the Schools, and hardly allow of being handed on to the public.

That all our knowledge begins with experience there can be no doubt. For how is it possible that the faculty of cognition should be awakened into exercise otherwise than by means of objects which affect our senses, and partly of them selves produce representations, partly rouse our powers of understanding into activity, to compare, to connect, or to separate these, and so to convert the raw material of our sensuous impressions into a knowledge of objects, which is called experience? In respect of time, therefore, no knowledge of ours is antecedent to experience, but begins with it. But though all our knowledge begins with experience, it by no means follows, that all arises out of experience. For, on the contrary, it is quite possible that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself (sensuous impressions giving merely the occasion)... It is, therefore, a question which requires close investigation, and is not to be answered at first sight,—whether there exists a knowledge altogether independent of experience, and even of all sensuous impressions? Knowledge of this kind is called à priori, in contradistinction to empirical knowledge which has its sources à posteriori, that is, in experience.

The light dove, cleaving the air in her free flight, and feeling its resistance, might imagine that its flight would be still easier in empty space.

All thought must, directly or indirectly, by way of certain characters, relate ultimately to intuitions, and therefore, with us, to sensibility, because in no other way can an object be given to us.

Thoughts without content are empty, intuitions without concepts are blind. The understanding can intuit nothing, the senses can think nothing. Only through their unison can knowledge arise.

The problem of establishing a perfect civic constitution is dependent upon the problem of a lawful external relation among states and cannot be solved without a solution of the latter problem.

This problem is the most difficult and the last to be solved by mankind.

The greatest problem for the human race, to the solution of which Nature drives man, is the achievement of a universal civic society which administers law among men.

The sham cause in physical influence consists in rashly assuming that the commerce of substance and transitive forces is sufficiently knowable from their mere existence. Hence it is not so much a system as rather the neglect of all philosophical system as a superfluity in the argument. Freeing the concept from this defect, we shall have a species of commerce alone deserving to be called real, and from which the whole constituting the world merits being called real, and not ideal or imaginary.

A whole from necessary substances is impossible. The whole, therefore, of substances is a whole of contingent things, and the world consists essentially of only contingent things.

Several actual worlds without one another are not, therefore, impossible by the very concept, as Wolf hastily concluded from the notion of a complex or multiplicity which he deemed sufficient to a whole, as such, but only on condition that there exist but one necessary cause of all things. If several are admitted, several worlds without one another will be possible in the strictest metaphysical sense.

If it were right to overstep a little the limits of apodictic certainty befitting metaphysics, it would seem worth while to trace out some things pertaining not merely to the laws but even to the causes of sensuous intuition, which are only intellectually knowable. Of course the human mind is not affected by external things, and the world does not lie open to its insight infinitely, except as far as itself together with all other things is sustained by the same infinite power of one. Hence it does not perceive external things but by the presence of the same common sustaining cause; and hence space, which is the universal and necessary condition of the joint presence of everything known sensuously, may be called the phenomenal omnipresence, for the cause of the universe is not present to all things and everything, as being in their places, but their places, that is the relations of the substances, are possible, because it is intimately present to all. Furthermore, since the possibility of the changes and successions of all things whose principle as far as sensuously known resides in the concept of time, supposes the continuous existence of the subject whose opposite states succeed; that whose states are in flux, lasting not, however, unless sustained by another; the concept of time as one infinite and immutable in which all things are and last, is the phenomenal eternity of the general cause} But it seems more cautious to hug the shore of the cognitions granted to us by the mediocrity of our intellect than to be carried out upon the high seas of such mystic investigations, like Malebranche, whose opinion that we see all things in God is pretty nearly what has here been expounded.

The use of the intellect in the sciences whose primitive concepts as well as axioms are given by sensuous intuition is only logical, that is, by it we only subordinate cognitions to one another according to their relative universality conformably to the principle of contradiction, phenomena to more general phenomena, and consequences of pure intuition to intuitive axioms. But in pure philosophy, such as metaphysics, in which the use of the intellect in respect to principles is real, that is to say, where the primary concept of things and relations and the very axioms are given originally by the pure intellect itself, and not being intuitions do not enjoy immunity from error, the method precedes the whole science, and whatever is attempted before its precepts are thoroughly discussed and firmly established is looked upon as rashly conceived and to be rejected among vain instances of mental playfulness.

The method of the science not being practiced much nowadays, except what logic prescribes to all sciences generally, that fitted for the peculiar nature of metaphysics being simply ignored, it is no wonder that those who everlastingly turn the Sisyphean stone of this inquiry do not seem so far to have made much progress. Though here I neither can nor will expatiate upon so important and extensive a subject, I shall briefly shadow forth what constitutes no despicable part of this method, namely, the infection between sensuous and intellectual cognition, not only as creeping in on those incautious in the application of principles, but even producing spurious principles under the appearance of axioms.

A spurious axiom of the first class is: Whatever is, is somewhere and sometime.

They cudgel their brains with absurd questions, such as, for instance, why God did not make the world many centuries earlier. They persuade themselves that it is easy to conceive, to be sure, how God may discern what is present, that is, what is actual in the time in which he is, but how He may foresee what is future, that is, what is actual in the time in which He is not yet, they deem an intellectual difficulty; as if the existence of the Necessary Being descended through all the moments of an imaginary time, and, having already exhausted a part of His duration, saw before Him the eternity He was yet to live simultaneously with the present events of the world. All these difficulties upon proper insight into the notion of time vanish like smoke.