"The belief in the continuance of things or processes between perceptions is not a blank act of faith, as would be the postulation of an external causal object for a single momentary percept; it may be said to be — not, indeed, rigorously verified — but strengthened by one of the most familiar of empirical facts — namely, that the same uniform causal sequences of natural events which may be observed within experience appear to go on in the same manner when not experienced. You build a fire in your grate of a certain quantity of coal, of a certain chemical composition. Whenever you remain in the room there occurs a typical succession of sensible phenomena according to an approximately regular schedule of clock-time; in, say, a half-hour the coal is half consumed; at the end of the hour the grate contains only ashes. If you build a fire of the same quantity of the same material under the same conditions, leave the room, and return after any given time has elaspsed, you get approximately the same sense-experiences as you would have had at the corresponding moment if you had remained in the room. You infer, therefore, that the fire has been burning as usual during your absence, and that being perceived is not a condition necessary for the occurence of the process. But a consistent idealist or phenomenalist cannot say this. He is committed to the proposition either that the fire has not been or, at all events, cannot legitimately be assumed to have been, burning when no one was perceiving it; his doctrine amounts to a gratuitous assumption of the universal jumpiness or temporal discontinuity of causal sequences. The most that he can admit — and he cannot admit less — is that fires and other natural process behave as if they went on when unobserved; if he desire to make this seem more intelligible, he may invoke some pre-established harmony, or resort to species of occasionalism — assuming that when you return to the room after an hour of God (as Descartes would have said) deceives you by putting into your mind a percept of a grate full of ashes, though these ashes are not the effects of any fire. But such 'explanations' of the facts are plainly arbitrary and far-fetched; they multiply types of causal agency beyond necessity. And to be content with a mere Philosphie des Als-Ob in such a case — to say that, although nothing at all that was like a fire or would have caused you to perceive a fire, if you had remained in the room, was really happening while you were absent, nevertheless all goes on as though the fire had been burning during that interval — this, surely, is a singularly strained and artificial notion, wholly foreign to the normal propensities of our intelligence."
—Arthur O. Lovejoy, The Revolt Against Dualism
"If realism is true, if we are animals trying to adjust ourselves to our environment, then our knowledge can be only the trial-and-error affair which I have depicted. If realism is true, our belief in the reality of the world, and in physical laws, cannot be demonstrable, or shown to be certain or 'reasonable' by any valid reasoning. In other words, if realism is right, we cannot expect or hope to have more than conjectural knowledge."
—Karl Popper, Realism and the Aim of Science
"Any person not 'sicklied o'er with the pale cast' of philosophical thought will regard it utterly perverse to believe that the everyday objects around us only become real through our experience of them. A normal man believes that the furniture in his bedroom is still there after he has left the room. The scientist who knows about evolution is firmly convinced of the reality of the external world: the sun shone for ages before there were eyes to see it....The notion that all this grandeur and probable infinity should only become reality when man, here today and gone tomorrow, happens to notice them, strikes a man of nature, be he farmer of biologist, as not only preposterous but completely blasphemous. In the face of such facts it is extraordinary that for centuries the wisest men, the greatest philosophers, above all Plato, should have been convinced idealists..."
---Konrad Lorenz, Behind the Mirror
"All knowledge, being faith in an object posited and partially described, is belief in substance, in the etymological sense of the word; it is belief in a thing or event subsisting in its own plane, and waiting for the light of knowledge to explore it eventually, and perhaps name or define it."
---George Santayana, Scepticism and Animal Faith
"It is, as a general rule, necessary always to distinguish between the concrete objective phenomenon and the form in which our mind perceives it: a form constituting another phenomenon which we may call subjective. To cite an everyday example: a straight stick immersed in water is the objective phenomenon. As we see it, this stick seems to be bent; and we would in fact say it was bent if we did not know it to be otherwise. This represents the subjective phenomenon...
"Because straight sticks on occassion appear bent, it must not be assumed that bent sticks do not in fact exist. The subjective phenomenon partly coincides with the facts and partly differs from it. Our ignorance of the facts, our passions and prejudices, the ideas in vogue in the societies in which we live, the events which powerfully affect us--all these with a thousand other circumstances conceal the truth from us and prevent us from getting an exact impression of the objective phenomenon giving rise to them. We are like a man that sees objects in a curved mirror; their outlines and proportions are to some degree altered. We must realize that most often we are aware only of this subjective phenomenon--i.e., the objective phenomenon in distorted form--knowing it either directly through investigation or the state of mind of the men witnessing a given event, or indirectly through the testimony of a historian who has conducted such an investigation. The problem which historical criticism has to resolve is therefore quite other than that involved in textual criticism. It amounts first and foremost to reconstituting the object itself, the image of which has been distorted."
--Vilfredo Pareto, Les Systèmes Socialistes