Scepticism
Scepticism is the philosophical view which holds that it is impossible to know anything with absolute certainty, or to know the world as it 'really' is. Scepticism began in the 5th century BC in Greece where certain philosophers expressed doubts about how certain we could be about our knowledge. Protagoras of Abdera (480-411 BC), for instance, is reported to have said that "man is the measure of all things" (i.e. that we make the world in our own image) and Gorgias (485-380 BC) that "nothing exists; if anything does exist, it cannot be known; if anything exists and can be known, it cannot be communicated". Many such thinkers arose from the group known as the Sophists.
Next came the Pyrrhonists, so called after Pyrrho of Elis, it's founder, who argued that since we can never know true reality we should refrain from making judgements. His pupil, Timon of Philius, followed this by adding that equally good arguments could be made for either side of any argument. The New Academy of the 2nd century BC, founded by Carneades (214-129 BC), taught only that some arguments were more probable than others. Later sceptics include Aenesidemus (1st century BC), who put forward ten arguments in support of the sceptical position and the Greek physician Sextus Empiricus (3rd century AD), who argued the use of common sense over abstract theory.
Scepticism was revived in the early modern period by Michel de Montaigne and Blaise Pascal. Its most extreme exponent was the Scottish empiricist philosopher David Hume (1711-1776), who argued that certain assumptions - such as the link between cause and effect, natural laws, the existence of God and the soul - were far from certain. Hume argued that there are only two kinds of reasoning: what he called probable and demonstrative. The German philosopher Immanuel Kant (1724-1804), influenced by Hume, set limits to human knowledge by arguing that certain things - such as if there was proof for God, or if the world had a beginning - did not make sense to be asked. The German philosopher Friedrich Nietzsche (1844-1900) argued that objective knowledge did not actually exist, and his scepticism influenced French Existentialists such as Jean-Paul Sartre (1905-1980). The American philosopher George Santayana (1863-1952), argued that all belief is irrational. Modern day philosophy still retains the influence of earlier sceptical thinkers.
Idealism
The word idealism is derived from the Greek word ἰδέα, which simply means something seen, or the look of something. Idealism refers to any philosophy that argues that reality is somehow dependent upon the mind rather than independent of it.
Idealism includes the principles that:
The everyday world of things and people is not the world as it really is but simply as it appears to be.
The best reflection of the world is not found in physical and mathematical categories but in terms of self-aware thought.
Thought is the relation of each experience to that which it expresses, rather than the imposition of ready-made answers.
The first prominent modern Western idealist in the metaphysical sense was George Berkeley. Berkeley argued that there is no deep distinction between mental states. Berkeley expressed this with the Latin formula esse est percipi: to be is to be perceived.
Forms of idealism were prevalent in philosophy from the 18th century to the early 20th century. Transcendental idealism, advocated by Immanuel Kant, is the view that there are limits on what can be understood. Kant wrote his Critique of Pure Reason (1781–1787) in an attempt to reconcile the conflicting approaches of rationalism and empiricism, and to establish a new groundwork for studying metaphysics. He maintained that things-in-themselves existed independently of our perceptions and judgments. Continuing his work, Johann Gottlieb Fichte and Friedrich Schelling dispensed with belief in the independent existence of the world, and created a thoroughgoing idealist philosophy.
The most notable work was G.W.F. Hegel's Phenomenology of Spirit, of 1807. Hegel admitted his ideas weren't new, but that all the previous philosophies had been incomplete. His goal was to correctly finish their job. His program of acceptance and reconciliation of contradictions is known as the "Hegelian dialectic". Philosophers in the Hegelian tradition include Ludwig Andreas Feuerbach, Karl Marx; Friedrich Engels; and the British idealists, notably T.H. Green, J.M.E. McTaggart, and F.H. Bradley. Few 20th century philosophers have embraced idealism. However, quite a few have embraced Hegelian dialectic. Immanuel Kant's "Copernican Turn" also remains an important philosophical concept today.
Idealism includes a few of the types:
Metaphysical Idealism: asserts the ideality of reality.
Epistemological Idealism: the mind can only hold that which it can perceive.
Subjective Idealism: (Berkeley) nothing exists except minds and spirits and their perceptions and thoughts.
Transcendental Idealism: (Kant) human self, or "transcendental ego," constructs knowledge out of sense impressions.
Critical Idealism: The name that Kant preferred for his approach.
Formalistic Idealism: another name for Transcendental Idealism.
Objective Idealism: Opposition to Berkeley's Subjective Idealism.
Aesthetic Idealism: (Schelling) variant of Objective Idealism.
Moral Idealism: (Fichte) variant of Objective Idealism.
Dialectical Idealism: (Hegel) variant of Objective Idealism.
Absolute Idealism: (Hegel) the real world is a reflection of the mind.
Kantian Idealism: Relatively recent view that seeks to go 'back to Kant'.
Neo-Kantian Idealists: View that seeks to progress from Kant onwards.
Theist Idealism: (Lotze) theory of the world ground, when all things find their unity.
Theist Absolutism: (Tennant) accepts traditional theological monotheism.
Three key types of Idealism:
Neo-Kantianism: We organize experience through mental constructs.
Rational Choice Theory: People make rational decisions.
Hermeneutics: Experience is internal. Positivism ignores this.
Idealism is opposed to many philosophies that stress material matter, including Empiricism, Positivism, Skepticism, Atheism and Materialism. It is closer to systems that emphasize meaning that is derived from thought, such as Rationalism. Overall, it is used as a container for other philosophies such as Phenomenology and Conventionalism that also oppose purely material viewpoints.