A. Plato's Objective idealism: The world of changing, material objects (the visible world) is merely a fleeting image of the intelligible world--what Plato calls the realm of the Forms. Physical objects are real only insofar as they are intelligible, but they can be intelligible only in terms of that which does not change. What makes a thing intelligible as a certain kind of thing cannot be constantly changing: otherwise, it could not be identified as that kind of thing, nor would it be that kind of thing. So a thing is what it is in virtue of something that is not changing. But since the visible world is constantly changing, it cannot be used as the basis for identifying what things are. There must be an intelligible (non-sensual) realm in terms of which physical things are said to exist intelligibly. That is the realm of the Forms.
Plato's allegory of the cave and his image of the divided line are intended to clarify exactly how the things we experience in the sensible, ordinary world (e.g., chairs, drawn triangles) are less real than (and depend on) the ideal models (or "Forms") on which they rely for their existence and in terms of which they are intelligible. Just as drawings, reflections, or copies of sensible objects are not as real as the sensible things on which they depend, so sensible things are not as real as the concepts in terms of which they are identifiable. Concepts that rely on sensual imagination for their intelligibility--for example, mathematical concepts such as triangularity--are more real than, say, triangular blocks of wood or drawings of triangles. But even though concepts that are based on sense experience are not limited to any particular expression and are unchanging, they are not as real as the Forms, which do not rely for their existence or intelligibility on anything sensual and changing.
Some Forms (e.g., chair-ness) are the ideal models in terms of which physical objects (e.g., chairs) exist and are intelligible. Other even higher Forms (e.g., equality, justice) provide the means by which not only physical objects but also activities, relations, and even lower Forms themselves are identifiable. The Forms are not abstractions or generalizations based on our sensual experience of physical objects; rather, we know physical objects as what they are by knowing them in terms of their Forms. As such, in order to know that a chair is a chair, we have to know what chair-ness is first, and that means that we cannot begin with sensible experience. Likewise, in order to know that two numbers are equal, or that an action is a just action, we have to know first what equality or justice is. But that already assumes we know what a number or action is; and that can only be known by appealing to lower Forms that rely for their intelligibility and existence on higher Forms. The highest Forms are themselves intelligible and exist ultimately in terms of the "super" Form, the Good.
B. Other Forms of Idealism (e.g., Hegel): Mind (or Mentality itself) is what makes the universe intelligible: in fact, Mind is the intelligibility of the universe, the means by which it is conceivable as some thing in the first place. All reality (including individual minds) is an expression of a universal or absolute Mind. Through Mind, evolution is not merely a process of random, chance variations, but rather is a process whose aim is revealed in the success (not simply, the survival) of the fittest. The universe has a purpose, goal, or direction: in short, it exhibits a teleology. Guiding the universe is a purposive mind, God.
C. Subjective idealism (Berkeley): reality is what we experience of things, for we have no idea what it would mean to talk about reality apart from the reality that we perceive: to be is to be perceived or to perceive. [Objection: this seems to make reality depend on the individual subject.]
Objections to Idealism in general:
2. Dualism (e.g., Descartes): reality is composed of immaterial, mental things (minds, ideas), physical things (bodies in motion), and combinations of the two (e.g., humans). In human beings, minds and bodies interact (exactly how this occurs is a bit of a question). Perhaps God intervenes to cause the interaction on the occasion when something happens in the body and then something happens in the mind, or vice versa; this is called occasionalism. Or maybe God has set up a pre-established harmony or parallel between mental and bodily events (G. W. Leibniz).
Objections:
3. The Anti-metaphysical Posture: some thinkers (positivists) believe that the metaphysical search for an ultimate principle of reality is itself meaningless because so-called metaphysical statements are neither true by definition nor statements that can be empirically verified. Others (e.g., Nietzsche) object that such a search imposes a false uniformity on a world that is experienced as always fragmented: why, they ask, should we think that there is an ultimate explanatory principle for reality? Such an expectation might itself be an instance of nothing other than a power-move, an attempt to impose one's will on the world and others under the guise of "revealing" the truth.
Objections: