It is important to note that were it not for the idea of necessary connection, there would be no reason to believe that a currently observed cause will produce an unseen effect in the future or that a currently observed effect was produced by an unseen cause in the past, for the mere fact that past instances of the cause and the effect were contiguous and temporally ordered in a certain way does not logically imply that present and future instances will display the same relations. (Such an inference could be justified only if one assumed a principle such as “instances, of which we have had no experience, must resemble those, of which we have had experience, and that the course of nature continues always uniformly the same.” The problem with that principle is that it too stands in need of justification, and the only possible justification is question-begging. That is, one could argue that present and future experience will resemble past experience, because in the past, present and future experience resembled past experience. But that argument clearly assumes what it sets out to prove.)
Hume offered a “skeptical solution” of the problem of the origin of the idea of necessary connection. According to him, it arises from the feeling of “determination” that is created in the mind when it experiences the first member of a pair of events that it is long accustomed to experiencing together. When the mind observes the moving billiard ball striking the stationary one, it is moved by force of habit and custom to form an idea of the movement of the stationary ball—i.e., to believe that the stationary ball will move. The feeling of being “carried along” in this process is the impression from which the idea of necessary connection is derived. Hume’s solution is “skeptical” in the sense that, though it accounts for the origins of the idea of necessary connection, it does not make the causal inferences any more rational than they were before. The solution explains why we are psychologically compelled to form beliefs about future effects and past causes, but it does not justify those beliefs logically. It remains true that our only evidence for these beliefs is our past experience of contiguity and temporal precedence. “All inferences from experience, therefore, are effects of custom, not of reasoning.” Thus it is that custom, not reason, is the great guide of life. Substance
From the time of Plato, one of the most basic notions in philosophy has been “substance”—that whose existence does not depend upon anything else. For Locke, the substance of an object is the hidden “substratum” in which the object’s properties inhere and on which they depend for their existence. One of the reasons for Hume’s importance in the history of philosophy is that he rejected that notion. In keeping with his strict empiricism, he held that the idea of substance, if it answers to anything genuine, must arise from experience. But what kind of experience can that be? By its proponents’ own definition, substance is that which underlies an object’s properties, including its sensible properties; it is therefore in principle unobservable. Hume concluded, “We have therefore no idea of substance, distinct from that of a collection of particular qualities, nor have we any other meaning when we either talk or reason concerning it.” Furthermore, the things that earlier philosophers had assumed were substances are in fact “nothing but a collection of simple ideas, that are united by the imagination, and have a particular name assigned to them.” Gold, to take Hume’s example, is nothing but the collection of the ideas of yellow, malleable, fusible, and so on. Even the mind, or the “self,” is only a “heap or collection of different perceptions united together by certain relations and suppos’d, tho’ falsely, to be endow’d with a perfect simplicity or identity.” That conclusion had important consequences for the problem of personal identity, to which Locke had devoted considerable attention, for if there is nothing to the mind but a collection of perceptions, then there is no self that perdures as the subject of those perceptions. Therefore, it does not make sense to speak of the subject of certain perceptions yesterday as the same self, or the same person, as the subject of certain perceptions today or in the future. There is no self or person there. Immanuel Kant
Idealism is often defined as the view that everything that exists is mental. In other words, everything is either a mind or dependent for its existence on a mind. Immanuel Kant was not strictly an idealist according to that definition. His doctrine of “transcendental idealism” held that all theoretical (i.e., scientific) knowledge is a mixture of what is given in sense experience and what is contributed by the mind. The contributions of the mind are necessary conditions for having any sense experience at all. They include the spatial and temporal “forms” in which physical objects appear, as well as various extremely general features that together give the experience an intelligible structure. Those features are imposed when the mind, in the act of forming a judgment about experience, brings the content of experience under one of the “pure concepts of the understanding.” Those concepts are unity, plurality, and totality; reality, negation, and limitation; inherence and subsistence, causality and dependence, and community (or reciprocity); and possibility, existence, and necessity. Among the more noteworthy of the mind’s contributions to experience is causality, which Hume asserted has no real existence.
Kant, ImmanuelImmanuel Kant, pencil portrait by Hans Veit Schnorr von Carolsfeld; in the Kupferstichkabinett, Dresden, Germany.Marburg—Art Reference Bureau/Art Resource, New York
His idealism notwithstanding, Kant also believed that there exists a world independent of the mind and completely unknowable by it. That world consists of “things-in-themselves” (noumena), which do not exist in space and time and do not enter into causal relations. Because of his commitment to realism (minimal though it may have been), Kant was disturbed by Berkeley’s uncompromising idealism, which amounted to a denial of the existence of the external world. Kant found that doctrine incredible and rejected “the absurd conclusion that there can be appearance without anything that appears.”
Because Kant’s theory attributes to the mind many aspects of reality that earlier theories assumed are given in or derived from experience, it can be thought of as inverting the traditional relation in epistemology between the mind and the world. According to Kant, knowledge results not when the mind accommodates itself to the world but rather when the world conforms to the requirements of human sensibility and rationality. Kant compared his reorientation of epistemology to the Copernican revolution in astronomy, which placed the Sun rather than Earth at the centre of the universe.
According to Kant, the propositions that express human knowledge can be divided into three kinds (see above synthetic propositions" class="md-crosslink">Analytic and synthetic propositions): (1) analytic a priori propositions, such as “All bachelors are unmarried” and “All squares have four sides,” (2) synthetic a posteriori propositions, such as “The cat is on the mat” and “It is raining,” and (3) what he called “synthetic a priori” propositions, such as “Every event has a cause.” Although in the last kind of proposition the meaning of the predicate term is not contained in the meaning of the subject term, it is nevertheless possible to know the proposition independently of experience, because it expresses a condition imposed by the forms of sensibility. Nothing can be an object of experience unless it is experienced as having causes and effects. Kant stated that the main purpose of his doctrine of transcendental idealism was to show how such synthetic a priori propositions are possible.
Because human beings can experience the world only as a system that is bounded by space and time and completely determined by causal laws, it follows that they can have no theoretical (i.e., scientific) knowledge of anything that is inconsistent with such a realm or that by definition exists independently of it—including God, human freedom, and the immortality of the soul. Nevertheless, belief in those ideas is justified, according to Kant, because each is a necessary condition of our conceiving of ourselves as moral agents. G.W.F. Hegel