Idealism is a term used to describe a wide variety of philosophical positions. One can distinguish two general senses: A Platonic sense, and a modern sense. Idealism in the Platonic sense involves the claim that ideal things occupy a metaphysically privileged position in the universe. Idealism in the modern sense centers around the claim that at least large portions of reality (in particular, the experienced physical world) are metaphysically based in something mental (minds and their ideas or representations). Such a view stands in stark opposition with "materialist" views of reality, which claim that mental entities and properties are somehow based or grounded in non-mental, material entities and properties, of the sort with which physics is concerned (there are positions between the two extremes, such as dualism).
Though both types of idealism are first and foremost metaphysical positions, their proponents have typically tried to motivate them using epistemological considerations. Plato's concern with the ideal realm appears to have been largely motivated by questions concerning knowledge. Epistemological arguments play a central role in the defenses of modern idealism presented by the two most prominent idealists in modern Western philosophy: George Berkeley and Immanuel Kant. Though there are relations between the two types of idealism, this article will discuss them separately.
In Book VII of the Republic, Plato presented his famous "Allegory of the Cave," which stands as one of the most vivid images of Platonic idealism. Taken together with Book VI's sun metaphor, the picture that emerges is roughly as follows: Certain entities ("Forms") stand at the basis of reality. These things are ideal, not in a pictoral sense, but rather in the sense that they represent a sort of perfection. For example, the Form of the Good is the only entity that is entirely good. Other entities have some degree of goodness only by "participating" in the Form. Sensible objects have the properties they do participating imperfectly in a large number of Forms. This "participation" makes them somehow less real than the Forms, so that Plato describes them as mere shadows or reflections. Throughout the relevant discussion, Plato is clear that the metaphysical relation between sensible objects and Forms perfectly parallels (and, it is safe to assume, was inspired by) the epistemic relations between perceptual awareness of sensory particulars and intellectual awareness of abstract universals.
In the Republic, the relation of the Forms to the rest of reality received little more than a metaphorical explanation. The Forms were somehow (perhaps causally) responsible for the sensible world, but Plato gave no suggestion that illumination was possible on that front. In his (probably later) dialogue Timaeus, however, Plato presented a creation story that suggested a picture more in line with most religious orthodoxy (both as Plato knew it, and as what it would become). In the Timaeus, the world is created when a powerful demiurge (meaning "craftsman") shapes the physical world in the images of the Forms, which act as blueprints.
The Timaeus was one of the most influential of Plato's works for the Christian Platonists. Heavily influenced by that account, Augustine rejected the idea that God merely shaped the world at some point in time, and rather held that God timelessly created the world. Such a timeless creation was in many ways closer to the picture originally presented in the Republic. Augustine also rejected the picture of the Forms as independent of and prior to God, instead locating such eternal archetypes in God alone.
Versions of such a view lasted even into the modern era. The great German philosopher Leibniz held that God's understanding contained ideas of all possible things, and that his act of creation was simply him actualizing the combination of things that he knew to be best.
In the first section of his 1783 work, Prolegomena to Any Future Metaphysics, Kant defined "genuine" idealism as consisting in the assertion that, "there are none but thinking beings; all other things which we believe are perceived in intuitions are nothing but representations in the thinking beings, to which no object external to them corresponds" (4:288-89 in the Akademie edition). The view described here applies as well to Leibniz as to Berkeley. It involves a sweeping claim about the nature of reality—namely, that the very notion of something entirely non-mental existing is either incoherent (Berkeley) or else cannot survive philosophical reflection (Leibniz).
Kant offered this definition, however, in order to distance himself from such positions (when writing the Prolegomena, he was reeling from reviews of his 1781 Critique of Pure Reason which charged him with merely restating Berkeley's position). His view, which he described as "transcendental" or "critical" idealism (4:293-94), did not involve the claim that all non-mental things must exist in representations.
The distinction Kant aimed to draw can be turned into a useful general point. It is clearest to understand the term "idealism" in a relative sense and an absolute sense. In the relative sense, a philosopher is an idealist about a certain sort of entity or property, where this simply means that she believes that the existence and nature of that entity or property ultimately reduces to facts about minds and their representations. Given this, certain forms of idealism should be generally accepted—for instance, we might be idealists about a certain fictional character. Kant, then, was an idealist about a certain set of properties (including space and time), but not about others (for instance, the property of being able to affect other entities).
The absolute sense of "idealism," then, is relative idealism about all entities and properties. This is then a much stronger position, and one that cannot be conclusively argued for one entity or property at a time.
Inspired by the work of the French philosopher and theologian Nicolas Malebranche, the Irish Bishop George Berkeley believed that philosophical positions that posited absolutely non-mental entities in the universe (in particular, Cartesian material substance) were responsible for the spread of atheism and skepticism across Europe in the seventeenth and eighteenth centuries. According to a philosophical picture such as that advanced by John Locke, material substance was the crucial aspect of the physical world, and was responsible for causing representations in the mind. It could not, however, be directly perceived, and could only be known indirectly through the representations it caused.
But if material substance was at the core of physical reality and could not be directly known, then, Berkeley believed, it was inevitable that people would come to doubt whether it existed, and thereby come to question the reality of the world of everyday objects. Worse, in his view, this view described a universe that seemed capable of operating independently of God. Were people to become convinced of such a picture, it was inevitable that they would come to wonder if they had any reason for believing in God at all.
On the other hand, if people believed (1) that all that existed were minds and their representations, (2) that the world of everyday objects was simply composed of representations, and (3) that most of their representations were directly caused by God, then the source of those temptations towards skepticism and atheism would dry up.
In his two major works, the Principles of Human Knowledge (1710) and Three Dialogues Between Hylas and Philonous (1713), Berkeley presented two general arguments for his idealism: The first based on the differing representations we have of supposedly unchanging objects, and the second based on the very conceivability of something non-mental.
The first general argument might be schematized as follows: Our perceptions of objects change with changes in us (e.g. objects appear different shapes from different perspective angles), but, on the view that there exists some non-mental material substance, the underlying substance needn't change with (e.g.) changes in our position. Yet there is no non-arbitrary way of determining which of those changing perceptions is correct, in the sense of revealing the true nature of the object. Because those perceptions are often incompatible, they cannot all reveal the nature of the object, but since they are all on par, the only reasonable conclusion is that none of them do. But that, Berkeley claimed, is obviously absurd; of course human perceptions say something about the nature of the object. That's why people use their perception in the first place. Given this, he thought that the only reasonable alternative was to identify the object with one's perceptions of it, thereby allowing one direct epistemic access to it (this relied on the uncontroversial assumption that people have direct access to their perceptions).
The first argument, however, is not nearly strong enough to establish absolute idealism, which was Berkeley's aim. It leaves open the possibility that the objects people perceive have an unknown reality, as well as the possibility that there might be unperceivable and non-mental objects. To rule out those possibilities, Berkeley presented another line of argument. Accepting a strong form of empiricism, Berkeley claimed that the only understanding of "existence" one can have must be one derived from his experiences. Human experiences, however, are all of one's own mind and one's own representations. But in that case, the only meaning that existence can have is "to have a representation or be a representation." Material substance, however, was supposed to be something that was neither a representation nor a possessor of representations. The conclusion is that "material substance exists" is in fact a contradiction.
Berkeley's second argument (presented above) relied heavily on the claim that all of one's meaningful thoughts must be based in direct experience. While this thought has appealed to some philosophers (perhaps most notably in the twentieth century, the logical positivists), it strikes most people as highly problematic. For instance, people seem to be able to think thoughts with universal and necessary content (for instance, all events have a cause), even though experience alone seems insufficient to yield ideas of universality or necessity.
Motivated by just such thoughts, Kant rejected the strong empiricist assumptions that underlay Berkeley's most radical arguments. Nevertheless, in his Critique of Pure Reason, he advanced arguments for forms of relative idealism about almost all qualities of objects, including their spatiality, temporality, and all sensible qualities.
With respect to space and time, Kant believed that some form of idealism was required to explain the vast store of a priori knowledge people have concerning the spatial and temporal properties of objects (the clearest example being geometry). How, Kant wondered, could people know, as they doubtless do, that all objects they could encounter have a spatial relation to each other and can be described mathematically? After all, people have experienced only a minute fraction of what exists, so they are hardly in a place to draw any inductive inference to such a conclusion. The only way one could explain this bulk of necessary, universal knowledge, Kant believed, was if space and time only existed as representations in the mind that one imposes on objects she encounters.
Nevertheless, Kant was clear that this does not mean that the objects people encounter only exist in their representations. The objects exist on their own—it is rather a certain set of their properties that are ideal. They almost certainly have other properties beyond those people encounter, and those properties needn't have any relation to anything mental. Kant often puts this distinction in terms of a contrast between "things as they appear to us" and "things as they are in themselves." By emphasizing ignorance of how things are in themselves, Kant hoped to rule out the possibility that natural science (which has to do only with things as they appear) could disprove the existence of freedom of the will or the existence of God.
Kant's idealism was enormously influential. Many of his successors, however, believed that his insistence on the existence of things in themselves showed that he had not taken his own insight concerning knowledge seriously enough. If knowledge only concerns representations, they thought, how could one even know the possibility of something outside of those representations? How could that even make sense? In response to these worries, absolute idealism surfaced again in Germany in the work of such thinkers as Fichte and Hegel. This issued in the era known as "German Idealism."
Fichte and Hegel's views are present in some of the most difficult pieces of philosophy ever produced (e.g. Fichte's Theory of Science or Wissenschaftslehre and Hegel's Phenomenology of Spirit). Yet the core idea is relatively simple: Whereas Berkeley believed that some supremely powerful mind (God) was needed to explain the varied perceptions humans experience, and Kant explained experience in terms of interactions with things whose inner natures humans were unaware of, Fichte (in his later work) and Hegel believed that such explanations could come from features internal to the force that manifests itself in finite minds (some sort of general mental force).
The advantage of such a move was that there was no longer an appeal to anything as supernatural as God or things in themselves. The disadvantage is the resulting difficulty in explaining how features of one's own mind could possibly account for the wildly varying and deeply complex set of representations we experience.
Despite this daunting philosophical challenge, the philosophical picture proposed by the German Idealists was extremely influential. It enjoyed a surge of popularity in English speaking countries in the late nineteenth and early twentieth centuries, as present in such figures as F.H. Bradley, J.M.E. McTaggart, and Josiah Royce.
The most natural response to idealism is that it violates some tenet of common sense. Berkeley was well aware of this, and spent much of his Three Dialogues attempted to argue to the contrary.
Yet a sustained philosophical attack on idealism was made (largely in response to Hegelian idealism) by the British philosopher G. E. Moore in the early twentieth century (Bertrand Russell made a parallel attack). Moore directly attacked that essential assumption of idealism, that what people are directly aware of are their representations. Instead, Moore proposed that people should understand the objects of their thoughts to be propositions, where propositions can be understood as states of affairs constituted by genuinely non-mental objects in the world. Such a picture has become the dominant one in contemporary analytic philosophy, and idealism is not often counted as a viably philosophical position. Nevertheless, defenders of idealism may well note that Moore's alternative picture is no more self-evident than the picture it meant to replace, so that the matter is far from settled.
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