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Rationalism and empiricism
Rationalism is any view emphasizing the role or importance of human reason. Extreme rationalism tries to base all knowledge on reason alone. Rationalism typically starts from premises that cannot coherently be denied, then attempts by logical steps to deduce every possible object of knowledge.
Modern rationalism begins with Descartes. Reflection on the nature of perceptual experience, as well as scientific discoveries in physiology and optics, led Descartes (and also Locke) to the view that we are directly aware of ideas, rather than objects. This view gave rise to three questions:
1. Is an idea a true copy of the real thing that it represents? Sensation is not a direct interaction between bodily objects and our sense, but is a physiological process involving representation (for example, an image on the retina). Locke thought that a 'secondary quality' such as a sensation of green could in no way resemble the arrangement of particles in matter that go to produce this sensation, although he thought that 'primary qualities' such as shape, size, number, were really in objects.
2. How can physical objects such as chairs and tables, or even physiological processes in the brain, give rise to mental items such as ideas? This is part of what became known as the mind-body problem.
3. If all the contents of awareness are ideas, how can we know that anything exists apart from ideas?
Descartes tried to address the last problem by reason. He began, echoing Parmenides, with a principle that he thought could not coherently be denied: I think, therefore I am (often given in his original Latin: Cogito ergo sum). From this principle, Descartes went on to construct a complete system of knowledge (which involves proving the existence of God, using, among other means, a version of the ontological argument). His view that reason alone could yield substantial truths about reality strongly influenced those philosophers usually considered modern rationalists (such as Baruch Spinoza, Gottfried Leibniz, and Christian Wolff), while provoking criticism from other philosophers who have retrospectively come to be grouped together as empiricists.
Empiricism, in contrast to rationalism, downplays or dismisses the ability of reason alone to yield knowledge of the world, preferring to base any knowledge we have on our senses. This dates back to the concept of tabula rasa (unscribed tablet) implicit in Aristotle's On the Soul, described more explicitly in Avicenna's The Book of Healing. John Locke propounded the classic empiricist view in An Essay Concerning Human Understanding in 1689, developing a form of naturalism and empiricism on roughly scientific (and Newtonian) principles.
During this era, religious ideas played a mixed role in the struggles that preoccupied secular philosophy. Bishop Berkeley's famous idealist refutation of key tenets of Isaac Newton is a case of an Enlightenment philosopher who drew substantially from religious ideas.
British empiricism
Francis Bacon
Bacon did not propose an actual philosophy, but rather a method of developing philosophy. Before beginning this induction, the inquirer is to free his or her mind from certain false notions or tendencies which distort the truth. These are called "Idols", and are of four kinds: "Idols of the Tribe", which are common to the race; "Idols of the Den", which are peculiar to the individual; "Idols of the Marketplace" , coming from the misuse of language; and "Idols of the Theatre", which result from an abuse of authority. The end of induction is the discovery of forms, the ways in which natural phenomena occur, the causes from which they proceed.
Derived through use of his methods, Bacon explicates his somewhat fragmentary ethical system in the seventh and eighth books of his De augmentis scientiarum (1623). He distinguishes between duty to the community, an ethical matter, and duty to God, a religious matter. Bacon claimed that [1] any moral action is the action of the human will, which is governed by belief and spurred on by the passions; [2] good habit is what aids men in directing their will toward the good; [3] no universal rules can be made, as both situations and men's characters differ.
Bacon contrasted the new approach of the development of science with that of the Middle Ages. He said:
"Men have sought to make a world from their own conception and to draw from their own minds all the material which they employed, but if, instead of doing so, they had consulted experience and observation, they would have the facts and not opinions to reason about, and might have ultimately arrived at the knowledge of the laws which govern the material world."
Earlier concepts of the existence of "innate ideas" were the subject of debate between the Continental rationalists and the British empiricists in the 17th century through the late 18th century. John Locke, George Berkeley, and David Hume were the primary exponents of empiricism.
Responding to the continental "rationalism" most prominently defended by René Descartes (a type of philosophical approach which should not be confused with rationalism generally), John Locke (1632-1704), writing in the late 17th century, in his An Essay Concerning Human Understanding (1689), proposed a very influential view wherein the only knowledge humans can have is a posteriori, i.e., based upon experience. Locke is famously attributed with holding the proposition that the human mind is a tabula rasa, a "blank tablet," in Locke's words "white paper," on which is written the experiences derived from sense impressions as a person's life proceeds. There are two sources of our ideas: sensation and reflection. In both cases, a distinction is made between simple and complex ideas. The former are unanalysable, and are broken down into primary and secondary qualities. Complex ideas are those which combine simple ones and are divided into substances, modes and relations. According to Locke, our knowledge of things is a perception of ideas that are in accordance or discordance with each other, which is very different from the quest for certainty of Descartes.
A generation later, the Irish Anglican bishop, George Berkeley (1685-1753), determined that Locke's view immediately opened a door that would lead to eventual atheism. In response to Locke, he put forth in his Treatise Concerning the Principles of Human Knowledge (1710) a different, very extreme form of empiricism in which things only exist either as a result of their being perceived, or by virtue of the fact that they are an entity doing the perceiving. (For Berkeley, God fills in for humans by doing the perceiving whenever humans are not around to do it). In his text Alciphron, Berkeley maintained that any order humans may see in nature is the language or handwriting of God. Berkeley's approach to empiricism would later come to be called subjective idealism.
The Scottish philosopher David Hume (1711-1776) added to the empiricist viewpoint an extreme skepticism that he brought to bear against the accumulated arguments and counterarguments of Descartes, Locke and Berkeley, among others. Hume argued in keeping with the empiricist view that all knowledge derives from sense experience. In particular, he divided all of human knowledge into two categories: relations of ideas and matters of fact. Mathematical and logical propositions (e.g. "that the square of the hypotenuse is equal to the sum of the squares of the two sides") are examples of the first, while propositions involving some contingent observation of the world (e.g. "the sun rises in the East") are examples of the second. All of people's "ideas", in turn, are derived from their "impressions". For Hume, an "impression" corresponds roughly with what we call a sensation. To remember or to imagine such impressions is to have an "idea". Ideas are therefore the faint copies of sensations.
Via his skeptical arguments (which became famous for the tenacity of their logic) he maintained that all knowledge, even the most basic beliefs about the natural world, cannot be conclusively established by reason. Rather, he maintained, our beliefs are more a result of accumulated habits, developed in response to accumulated sense experiences. Among his many arguments Hume also added another important slant to the debate about scientific method - that of the problem of induction. Hume argued that it requires inductive reasoning to arrive at the premises for the principle of inductive reasoning, and therefore the justification for inductive reasoning is a circular argument. Among Hume's conclusions regarding the problem of induction is that there is no certainty that the future will resemble the past. Thus, as a simple instance posed by Hume, we cannot know with certainty by inductive reasoning that the sun will continue to rise in the East, but instead come to expect it to do so because it has repeatedly done so in the past.
Hume concluded that such things as belief in an external world and belief in the existence of the self were not rationally justifiable. According to Hume these beliefs were to be accepted nonetheless because of their profound basis in instinct and custom. Hume's lasting legacy, however, was the doubt that his skeptical arguments cast on the legitimacy of inductive reasoning, allowing many skeptics who followed to cast similar doubt.
Rationalism
René Descartes (1596-1650)
Descartes thought that only knowledge of eternal truths - including the truths of mathematics, and the epistemological and metaphysical foundations of the sciences - could be attained by reason alone; other knowledge, the knowledge of physics, required experience of the world, aided by the scientific method. He also argued that although dreams appear as real as sense experience, these dreams cannot provide persons with knowledge. Also, since conscious sense experience can be the cause of illusions, then sense experience itself can be doubtable. As a result, Descartes deduced that a rational pursuit of truth should doubt every belief about reality. He elaborated these beliefs in such works as Discourse on Method, Meditations on First Philosophy, and Principles of Philosophy. Descartes developed a method to attain truths according to which nothing which cannot be recognised by the intellect (or reason) can be classified as knowledge. These truths are gained "without any sensory experience", according to Descartes. Truths that are attained by reason are to be broken down into elements which intuition can grasp, which, through a purely deductive process, will result in clear truths about reality.
Descartes therefore argued, as a result of his method, that reason alone determined knowledge, and that this could be done independently of the senses. For instance, his famous dictum, cogito ergo sum, is a conclusion reached a priori and not through an inference from experience. This was, for Descartes, an irrefutable principle upon which to ground all forms of other knowledge. Descartes posited a metaphysical dualism, distinguishing between the substances of the human body ("res extensa") and the mind or soul ("res cogitans") . This crucial distinction would be left unresolved and lead to what is known as the mind-body problem, since the two substances in the Cartesian system are independent of each other and irreducible.
Baruch or Benedict de Spinoza (November 24, 1632 - February 21, 1677) was a Dutch philosopher . Substance, Attribute and Mode.
"These are the fundamental concepts with which Spinoza sets forth a vision of Being, illuminated by his awareness of God. They may seem strange at first sight. To the question "What is?" he replies: "Substance, its attributes and modes".
Spinoza's system imparted order and unity to the tradition of radical thought, offering powerful weapons for prevailing against "received authority." As a youth he first subscribed to Descartes's dualistic belief that body and mind are two separate substances, but later changed his view and asserted that they were not separate, being a single identity. He contended that everything that exists in Nature/Universe is one Reality (substance) and there is only one set of rules governing the whole of the reality which surrounds us and of which we are part. Spinoza viewed God and Nature as two names for the same reality, namely the single substance (meaning "that which stands beneath" rather than "matter") that is the basis of the universe and of which all lesser "entities" are actually modes or modifications, that all things are determined by Nature to exist and cause effects, and that the complex chain of cause and effect is only understood in part. That humans presume themselves to have free will, he argues, is a result of their awareness of appetites while being unable to understand the reasons why they want and act as they do.
Spinoza was a thoroughgoing determinist who held that absolutely everything that happens occurs through the operation of necessity. For him, even human behaviour is fully determined, with freedom being our capacity to know we are determined and to understand why we act as we do. So freedom is not the possibility to say "no" to what happens to us but the possibility to say "yes" and fully understand why things should necessarily happen that way.
Gottfried Leibniz (1646-1716)
Leibniz was the last of the great Rationalists, who contributed heavily to other fields such as mathematics. His system however was not developed independently of these advances. Leibniz rejected Cartesian dualism, and denied the existence of a material world. In Leibniz's view there are infinitely many simple substances, which he called "monads".
Leibniz developed his theory of monads in response to both Descartes and Spinoza. In rejecting this response he was forced to arrive at his own solution. Monads are the fundamental unit of reality, according to Leibniz, constituting both inanimate and animate things. These units of reality represent the universe, though they are not subject to the laws of causality or space (which he called "well-founded phenomena"). Leibniz therefore introduced his principle of pre-established harmony, in order to account for apparent causality in the world.