From The Philosophy of the Future by Ludwig Feuerbach, 1843

The self-realisation of the idea means that it negates itself and ceases to be a mere idea. What is then this not-thinking, that which is differentiated from thinking? It is the sensuous. The self-realisation of the idea means, accordingly, that it makes itself into an object of the senses. The reality of the idea is thus sensation. But reality is the truth of the idea; thus, sensation is the truth of the idea. Precisely so we managed to make sensation a predicate and the idea or thought a subject. But why, then, does the idea represent itself in sensation? Why is it not true when it is not real, that is, sensuous? Is not its truth made, therefore, dependent on sensation? Is not meaning and worth granted to the sensuous for itself, disregarding the fact that it is the reality of the idea? If sensation for itself is nothing, of what need is it to the idea? If only the idea gives value and content to sensation, then sensation is a pure luxury and a trifle; it is only an illusion that the idea presents to itself. But it is not so. The idea is required to realise itself and represent itself in sensation only because, unknowing to the idea, reality and sensation, independent of the idea, are presupposed as the truth. The idea proves its worth through sensation; how would this be possible if sensation were not unconsciously accepted as the truth? Because, however, one starts consciously with the truth of the idea, the truth of sensation is expressed only afterward, and sensation is made only into an attribute of the idea. This is, however, a contradiction, for it is only an attribute and still it gives truth to the idea; namely, it is simultaneously the main thing and an accessory, essence and accident. We save ourselves from this contradiction only if we make the real, that is, the sensuous, into its own subject and give it an absolutely independent, divine, and primary meaning which is not first derived from the idea.

The real in its reality or taken as real is the real as an object of the senses; it is the sensuous. Truth, reality, and sensation are identical. Only a sensuous being is a true and real being. Only through the senses, and not through thought for itself, is an object given in a true sense. The object that is given in thought or that is identical with thought is only idea.

From Critique of Hegel's Philosophy of Right, by Karl Marx 1843

In regard to the former, both North and South Poles are poles; their essence is identical. In the same way both female and male gender are of one species, one nature, i.e., human nature. North and South Poles are opposed determinations of one essence, the variation of one essence brought to its highest degree of development. They are the differentiated essence. They are what they are only as differentiated determinations; that is, each is this differentiated determination of the one same essence. Truly in real extremes would be Pole and non-Pole, human and non-human gender. Difference here is one of existence, whereas there [i.e., in the case of Pole and non-Pole, etc.,] difference is one of essence, i.e., the difference between two essences. in regard to the second [i.e. where each extreme is its other extreme], the chief characteristic lies in the fact that a concept (existence, etc.) is taken abstractly, and that it does not have significance as independent but rather as an abstraction from another, and only as this abstraction. Thus, for example, spirit is only the abstraction from matter. It is evident that precisely because this form is to be the content of the concept, its real essence is rather the abstract opposite, i.e., the object from which it abstracts taken in its abstraction - in this case, abstract materialism.

From The Concept of Dread, by Søren Kierkegaard, 1844

The fact that science, fully as much as poetry and art, assumes a mood both on the part of the producer and on the part of the recipient, that an error in modulation is just as disturbing as an error in the exposition of thought, has been entirely forgotten in our age, when people have altogether forgotten inwardness and appropriation with the characteristic joy they prompt at the thought of all the glory one believed one possessed or through cupidity had renounced, like the dog which preferred the shadow. However, every error begets its own enemy. An error of thought has outside of it as its enemy, dialectics; the absence of mood or its falsification has outside of it its enemy, the comical.

From Prejudices of Philosophers, Friedrich Nietzsche, 1885

THE Will to Truth, which is to tempt us to many a hazardous enterprise, the famous Truthfulness of which all philosophers have hitherto spoken with respect, what questions has this Will to Truth not laid before us! What strange, perplexing, questionable questions! It is already a long story; yet it seems as if it were hardly commenced. Is it any wonder if we at last grow distrustful, lose patience, and turn impatiently away? That this Sphinx teaches us at last to ask questions ourselves? Who is it really that puts questions to us here? What really is this "Will to Truth" in us? In fact we made a long halt at the question as to the origin of this Will - until at last we came to an absolute standstill before a yet more fundamental question. We inquired about the value of this Will. Granted that we want the truth: why not rather untruth?

From Analysis of Sensations, by Ernst Mach 1886

Colours, sounds, temperatures, pressures, spaces, times, and so forth, are connected with one another in manifold ways; and with them are associated dispositions of mind, feelings, and volitions. Out of this fabric, that which is relatively more fixed and permanent stands prominently forth, engraves itself on the memory, and expresses itself in language. Relatively greater permanency is exhibited, first, by certain complexes of colours, sounds, pressures, and so forth, functionally connected in time and space, which therefore receive special names, and are called bodies. Absolutely permanent such complexes are not. ...

The useful habit of designating such relatively permanent compounds by single names, and of apprehending them by single thoughts, without going to the trouble each time of an analysis of their component parts, is apt to come into strange conflict with the tendency to isolate the component parts. The vague image which we have of a given permanent complex, being an image which does not perceptibly change when one or another of the component parts is taken away, seems to be something which exists in itself. Inasmuch as it is possible to take away singly every constituent part without destroying the capacity of the image to stand for the totality and to be recognised again, it is imagined that it is possible to subtract all the parts and to have something still remaining. Thus naturally arises the philosophical notion, at first impressive, but subsequently recognised as monstrous, of a "thing-in-itself," different from its "appearance," and unknowable.

Thing, body, matter, are nothing apart from the combinations of the elements, - the colours, sounds, and so forth - nothing apart from their so-called attributes.

From The Preface to the Philosophy of Right, by Hegel 1821

Only one word more concerning the desire to teach the world what it ought to be. For such a purpose philosophy at least always comes too late. Philosophy, as the thought of the world, does not appear until reality has completed its formative process, and made itself ready. History thus corroborates the teaching of the conception that only in the maturity of reality does the ideal appear as counterpart to the real, apprehends the real world in its substance, and shapes it into an intellectual kingdom. When philosophy paints its grey in grey, one form of life has become old, and by means of grey it cannot be rejuvenated, but only known. The owl of Minerva, takes its flight only when the shades of night are gathering.

From The Preface to the Philosophy of Right, by Hegel 1821

What is rational is real;
And what is real is rational.

Upon this conviction stand not philosophy only but even every unsophisticated consciousness. From it also proceeds the view now under contemplation that the spiritual universe is the natural. When reflection, feeling or whatever other form the subjective consciousness may assume, regards the present as vanity, and thinks itself to be beyond it and wiser, it finds itself in emptiness, and, as it has actuality only in the present, it is vanity throughout. Against the doctrine that the idea is a mere idea, figment or opinion, philosophy preserves the more profound view that nothing is real except the idea. Hence arises the effort to recognise in the temporal and transient the substance, which is immanent, and the eternal, which is present. The rational is synonymous with the idea, because in realising itself it passes into external existence. It thus appears in an endless wealth of forms, figures and phenomena. It wraps its kernel round with a robe of many colours, in which consciousness finds itself at home.

From The Philosophy of Right (s. 348), by Hegel 1821

All actions, including world-historical actions, culminate with individuals as subjects giving actuality to the substantial (see Remark to 279). They are the living instruments of what is in substance the deed of the world mind and they are therefore directly at one with that deed though it is concealed from them and is not their aim and object (see 344). For the deeds of the world mind, therefore, they receive no honour or thanks either from their contemporaries (see 344) or from public opinion in later ages. All that is vouchsafed to them by such opinion is undying fame in respect of the subjective form of their acts.

From The Philosophy of Right (addition to s. 258), by Hegel 1821

The state in and by itself is the ethical whole, the actualisation of freedom; and it is an absolute end of reason that freedom should be actual. The state is mind on earth and consciously realising itself there. In nature, on the other hand, mind actualises itself only as its own other, as mind asleep. Only when it is present in consciousness, when it knows itself as a really existent object, is it the state. In considering freedom, the starting-point must be not individuality, the single self-consciousness, but only the essence of self-consciousness; for whether man knows it or not, this essence is externally realised as a self-subsistent power in which single individuals are only moments. The march of God in the world, that is what the state is. The basis of the state is the power of reason actualising itself as will. In considering the Idea of the state, we must not have our eyes on particular states or on particular institutions. Instead we must consider the Idea, this actual God, by itself. On some principle or other, any state may be shown to be bad, this or that defect may be found in it; and yet, at any rate if one of the mature states of our epoch is in question, it has in it the moments essential to the existence of the state. But since it is easier to find defects than to understand the affirmative, we may readily fall into the mistake of looking at isolated aspects of the state and so forgetting its inward organic life. The state is no ideal work of art; it stands on earth and so in the sphere of caprice, chance, and error, and bad behaviour may disfigure it in many respects. But the ugliest of men, or a criminal, or an invalid, or a cripple, is still always a living man. The affirmative, life, subsists despite his defects, and it is this affirmative factor which is our theme here.

From What Pragmatism Means by William James, 1906

Our beliefs are really rules for action; to develop a thought's meaning, we need only determine what conduct it is fitted to produce: that conduct is for us its sole significance. ... To attain perfect clearness in our thoughts of an object, then, we need only consider what conceivable effects of a practical kind the object may involve - what sensations we are to expect from it, and what reactions we must prepare. ...

Aspirants to a philosophic religion turn, as a rule, more hopefully nowadays towards idealistic pantheism than towards the older dualistic theism, ...

But, ... the brand of pantheism offered is hard for them to assimilate if they are lovers of facts, or empirically minded. It is the absolutistic brand, spurning the dust and reared upon pure logic. It keeps no connection whatever with concreteness. Affirming the Absolute Mind, which is its substitute for God, to be the rational presupposition of all particulars of fact, whatever they may be, it remains supremely indifferent to what the particular facts in our world actually are. Be they what they may, the Absolute will father them. ...

Now pragmatism, devoted though she be to facts, has no such materialistic bias as ordinary empiricism labours under. Moreover, she has no objection whatever to the realising of abstractions, so long as you get about among particulars with their aid and they actually carry you somewhere. Interested in no conclusions but those which our minds and our experiences work out together, she has no a priori prejudices against theology. If theological ideas prove to have a value for concrete life, they will be true, for pragmatism, in the sense of being good for so much. For how much more they are true, will depend entirely on their relations to the other truths that also have to be acknowledged.

From Dialectical & Historical Materialism, by Josef Stalin 1938

Contrary to idealism, which regards the world as the embodiment of an "absolute idea," a "universal spirit," "consciousness," Marx's philosophical materialism holds that the world is by its very nature material, that the multifold phenomena of the world constitute different terms of matter in motion, that interconnection and interdependence of phenomena, as established by the dialectical method, are a law of the development of moving matter, and that the world develops in accordance with the laws of movement of matter and that it stands in no need of a "universal spirit."

Keynes (1920) vs. Friedman (1953) on Economic Theory

Pure Induction can be used to support the generalisation that the sun will rise every morning for the next million years, provided that with the experience we have actually had there are finite probabilities, however small, derived from some other source, first, in favour of the generalisation, and, second, in favour of the sun's not rising to-morrow assuming the generalisation to be false. Given these finite probabilities, obtained otherwise, however small, then the probability can be strengthened and can tend to increase towards certainty by the mere multiplication of instances provided that these instances are so far distinct that they are not inferable one from another. [John Maynard Keynes on Induction 1953]

A theory cannot be tested by comparing its "assumptions" directly with "reality." Indeed, there is no meaningful way in which this can be done. Complete "realism" is clearly unattainable, and the question whether a theory is realistic "enough" can be settled only by seeing whether it yields predictions that are good enough for the purpose in hand or that are better than predictions from alternative theories. Yet the belief that a theory can be tested by the realism of its assumptions independently of the accuracy of its predictions is widespread and the source of much of the perennial criticism of economic theory as unrealistic. Such criticism is largely irrelevant, and, in consequence, most attempts to reform economic theory that it has stimulated have been unsuccessful. [Milton Friedman on Economic Theory 1920]

From Rudolph Carnap, Philosophical Foundations of Physics, 1966

Proponents of the descriptive view remind us that unobservable entities have a habit of passing over into the observable realm as more powerful instruments of observation are developed. At one time, "virus" was a theoretical term. The same is true of "molecule". Ernst Mach was so opposed to thinking of a molecule as an existing "thing" that he once called it a "valueless image". Today, even atoms in a crystal lattice can be photographed by bombarding them with elementary particles; in a sense, the atom itself has become an observable. Defenders of this view argue that it is as reasonable to say that an atom "exists" as it is to say that a distant star, observable only as a faint spot of light on a long-exposed photographic plate, exists. There is, of course, no comparable way to observe an electron. But that is no reason for refusing to say it exists. Today, little is known about its structure; tomorrow a great deal may be known. It is as correct, say the advocates of the descriptive approach, to speak of an electron as an existing thing as it is to speak of apples and tables and galaxies as existing things.