Version 2, October 26, 2005
Modern epistemology took shape in the seventeenth century, after the recovery of the texts of ancient philosophy in the Renaissance had injected new life into philosophy. The "modern" period extends roughly from the middle of the seventeenth century to the middle of the nineteenth century, a period covering about 200 years. We will refer to the philosophers of this period, including the most influential epistemologists Descartes, Locke, Hume, and Kant, as "modern philosophers."
The prevailing philosophy of the late Middle Ages had been based on the writings of Aristotle. The modern philosophers led a full-scale revolt against Aristotelian philosophy, and that revolt had important repercussions for the theory of knowledge.
Recall that Aristotle's theory of scientific knowledge was centered on the universal. In effect, all knowledge is traced back to definitions which state the essence of things of a given kind. The modern philosophers found two main problems with the Aristotelian epistemology. The first was that experience seems to yield information about things beyond what is captured in the universal that defines a thing. If we want to know why an acorn grows into an oak tree, Aristotle's explanation that the acorn has in it potentially the form of a mature tree does not address the question of how the plant develops physically. The modern philosophers thought that the explanation lay in the "mechanism" to be found in the parts of the plant.
The second main problem is that appeal to definitional universals gives no role to mathematical descriptions of objects. Aristotle himself rarely applied mathematics to his explanation of physical change. Galileo Galilei, working in the early seventeenth century, was the first modern philosopher to apply rigorous mathematical analysis to physical problems. He battled valiantly to overcome the inertia of Aristotelian natural science, only to be censured by the Roman Catholic Church for claiming that the earth moves around the sun. This account of astronomical phenomena was deemed to be in conflict with certain passages in the Bible.
The rise of the mechanical, mathematical description of nature meant that Aristotelian epistemology had to be replaced by something new. The exact character of the replacement was the subject of much dispute.
One factor that was to shape the new epistemology was the rejection of Aristotle's explanation of the process of perception. On Aristotle's view, perception takes place through the transmission of "forms" from the object to the human mind, which in turn changes the "form" of the mind itself. The form is universal in that it is shared by other objects of its kind, which allows the "informing" of the mind to yield knowledge of the object.
Modern metaphysics virtually abolished Aristotelian forms. The modern philosophers returned to the basic approach of the ancient atomists, who held that perception takes place through the actions of imperceptible, indivisible, atoms. The modern philosophers preferred to call the tiny particles "corpuscles," which is a more generic term that does not imply indivisibility. Rather than forms, they held, it is corpuscles that are transmitted from perceived objects to the body when the object is perceived. These corpuscles in turn interact with corpuscles making up the human body, including the brain. The end-product is a kind of "image" which is in reality a physical configuration of corpuscles in the brain.
The main question of the descriptive epistemology of perception was how (or whether) the impressions generated by the action of corpuscles could give rise to knowledge of the objects from which they originated. A question of validation is whether the postulation of the existence of corpuscles amounts to knowledge that they exist.
The question of validation applies to the ancient atomists. As the atomists recognized, the atoms could be known only through the application of reason to the presentations of the senses. As we would now put it, any knowledge we have of them would be "theoretical" rather than "observational."
The heavenly bodies (sun, moon, planets, stars, comets) and their relative positions are objects of perception. Since the time of the ancient Greeks, there have been attempts to make theoretical claims about them. From the beginning, there have been divergent descriptions of their size, the material of which they are composed, the way they move with respect to one another, the nature of eclipses, etc.
In the sixteenth century, astronomy was dominated by two theoretical accounts of the heavens (which in fact were in conflict with each other). One, due to Aristotle, was physical. The sun, moon, stars, etc. are embedded in spheres composed of a heavenly element, and these spheres rotate with the earth at their center. The other, due to Ptolemy, was a purely geometrical descriptions of paths defined by compounded circles. What they had in common was the claim that the earth lies in the center of the celestial system and that all the heavenly bodies move around it. This geocentric system was challenged at the beginning of the modern period by Copernicus and Galileo.
The heliocentric astronomy of Copernicus and Galileo adopted the mathematical approach of Ptolemy. But it combined it with a physical account of the motions of the heavenly bodies (which is what got Galileo in trouble). One of the by-products of the new astronomy was the recognition of the importance of mathematics in theorizing more generally about the physical world. As the modern period unfolded, it became clear that the use of mathematical "models" (as we now say) of observed motions (both celestial and terrestrial) provides a way to give unified accounts of them. Descartes went so far as to say that there are "laws of nature" which have a precise mathematical form. Newton formulated a powerful set of laws, by means of which he provided a very convincing explanation of all observed forms of motion.
This development raised a number of important epistemological issues. Do the mathematical "laws" of nature apply to all physical objects, observed and unobserved, past, present, and future? How could we know that they do? What is the basis of our claims to knowledge about the truth of the underlying mathematical propositions themselves? Do the mathematical representations of observed changes describe things as they really are, or are they simply handy devices for organizing our perception of them?
Other important issues involve the nature of explanation. If we abandon the Aristotelian view explanation is the intuitive grasping of a universal and deduction from universal definitions, how exactly are we supposed to understand it? If explanations conflict, how are we to determine which is the best of the lot? And is the fact that an explanation is relatively good a sufficient backing for claims to know that what does the explaining (the "explanandum," as opposed to the "explanans" which is to be explained) describes reality accurately? If the motion of the earth around the sun is the best explanation of the apparent motion of the sun, do we thereby know that the earth moves around the sun? What do we do when our scientific explanations conflict with other beliefs that we have, such as religious beliefs?
We have described the methodological project as being concerned with determining how the investigation of knowledge is to be carried out. By the modern period, accounts of knowledge had become sufficiently mature that sharp differences in methodology had emerged.
As with Plato and the other ancient epistemolgists, knowledge was regarded by the modern philosophers as a property that human beings may have or lack. The modern philosophers were not interested merely in how we make attributions of knowledge. They primarily wanted to know whether we have knowledge and what precisely it is that we know.
One of the main methodological disputes in epistemology has its roots early in the modern period. On the one hand, the way the philosophers thought about knowledge was profoundly influenced by their conceptions of the natural capacities of human knowers. Descartes, for example, conducted extensive investigations into the physiology of perception and the processes, such as the behavior of light, which bring it about. As will be seen, a basic picture of the nature of physical reality in general was quite influential in the way the philosophers thought about knowledge.
But on the other hand, the period is famous for its "armchair" approach to thinking about knowledge. The very same Descartes who carefully examined the eyeballs of dead cows is also responsible for a method of investigation which is carried out through thinking in an isolated setting, disengaged from practical interaction with the world. And he is responsible for the notorious "problem of the external world." How can I tell, from a purely subjective perspective, whether there is a world at all beyond my own mind?
The other methodological issue that was discussed in the introductory module concerns how one is to begin the investigation of knowledge. The methodist starts with a preconception of what knowledge is and asks whether we know. The particularist starts with a preconception of what we know and constructs an account of knowledge which will conform to our knowledge claims.
Most of the modern epistemologists were methodists. This is why the task of showing that we have knowledge, particularly of the existence of the external world, was so central to their investigations. The problems they encountered engendered a reaction by a number of eighteenth-century Scottish philosophers, most prominently Thomas Reid. These philosophers of "common sense" held that any account of knowledge that ends up in skepticism is, for that very reason, to be rejected. This view was harshly rejected by Immanuel Kant, and it lay dormant until revived in the early twentieth century by G. E. Moore.
The analytic project is the attempt to understand precisely what knowledge is. At the beginning of the period, the dominant account was based on the one given by Aristotle. He was primarily interested in "scientific knowledge," for which he set up the highest possible standard. Modern philosophers set high standards for knowledge as well, with the result that they had a difficult time battling skepticism.
The Port-Royal Logic
Although much was written about knowledge in the modern period, there were not many clear analyses of the concept. One of the more detailed attempts can be found in the 1662 book The Art of Thinking (more commonly referred to as "The Port-Royal Logic) by Antoine Arnauld. This account is broadly Aristotelian in its presentation, but it is extremely permissive with respect to what it counts as knowledge.
According to Arnauld, some propositions ("maxims") can be known to be true in itself by "intellection," "when the evidence offered by the maxim suffices to convince us of its truth" (Part IV, Chapter 1). Our knowledge of "first principles" is of this sort.
Other propositions are not self-evident in this way, and they are accepted as true either on the basis of authority or on the basis of reason. Authority-induced assent is called "faith." The use of reason either produces complete conviction or it does not. In the latter case, it is called "opinion."
If complete conviction is produced, there is (in an extremely weak sense) knowledge. It may be through an apparent reason or through genuine reason. In the former case, the conviction is due to a "lack of attention." If the maxim itself is false, then "the knower is in error," and even if it is true, "the knower has at the very least judged rashly."
The "knower" is said to have "understanding" in the case where "the reason is a genuine one, recognized as such by a fairly long and minute scrutiny as well as by a strong sense of persuasion and a vivid and penetrating quality of clarity."
Immanuel Kant, writing some 100 years later, gave a similar classification, distinguishing knowledge from opinion and faith. They are all levels of assent. Assent always has a subjective cause, a reason why we assent, and it may have an objective basis in reality. When we assent to something without conviction, the subjective cause is said to be "insufficient," and we hold an "opinion." If the subjective cause is sufficient to induce conviction, yet the person realizes that its objective basis is insufficient to establish its truth, there is "faith."
Knowledge is "assent that is sufficient both subjectively and objectively" (Critique of Pure Reason, Part II, Chapter II, Section III). The difference between knowledge and faith lies in the fact that assent is valid for everyone when one has knowledge, but is only valid for one's self when one has faith.
Writing in the first part of the seventeenth century, Descartes, in an unpublished work, declared that "All knowledge [scientia] is certain and evident cognition [cognitio]" (Rules for the Direction of the Mind, Rule Two). Certainty, in turn, is freedom from doubt. We shall later consider Descartes's epistemic standards for certainty.
Cognitions differ in their degree of certainty. Descartes distinguished between "absolute" and "moral certainty" (Principles of Philosophy Part Two, Article 206). We are absolutely certain "when we believe that it is wholly impossible that something should be otherwise than we judge it to be" (Principles of Philosophy Part Two, Article 206). We have moral certainty, which is not sufficient for scientia, when we do not normally doubt the truth of what we believe, though we recognize that it may be false.
Moral certainty is sufficient for the practical conduct of life. If we were to hold out for scientia before conducting our affairs, we would be paralyzed. Perhaps more importantly for Descartes, he could claim moral certainty with respect to his scientific theories of the world. It was in the context of justifying the uncertain hypotheses made in the conduct of science that he introduced the distinction.
Other "rationalist" philosophers such as Spinoza and Leibniz developed accounts of levels of certainty of cognitions (see Spinoza's Treatise on the Emendation of the Intellect and Leibniz's "Meditations on Knowledge, Truth, and Ideas"). But for them, as for Descartes, the prize was absolute certainty.
John Locke defined knowledge in an unusual way that we will not discuss here. Like Descartes, Locke held that knowledge must be evident and certain, and he also followed Descartes by distinguishing between degrees of certainty (An Essay Concerning Human Understanding, Book IV, Chapter II). Locke, however, claimed that knowledge does not require absolute certainty, but only a subjective kind of certainty. Specifically, Locke held that we can have knowledge of the existence of objects which are presently perceived. The reason he explicitly called this "knowledge" is that we are unable to bring ourselves to doubt in these cases. What falls short of this standard is called "opinion" or "probability."
David Hume wrote of the "assurance" we have in knowledge. He did not specify the degree of assurance required for knowledge, but it appears that he had Cartesian "absolute certainty" in mind (A Treatise of Human Nature, Book I, Part III, Section 11). He distinguished a level of assurance he called "proof," which is "entirely free from doubt and uncertainty." This is contrasted with probability proper, which is attended with uncertainty.
The accounts of knowledge handed down by the most influential modern philosophers set the tone for their epistemological investigations. Because they embraced very high standards of knowledge, they were constrained to limit the scope of knowledge severely. Only Locke allowed that we can have knowledge of particular facts about the external world. Descartes and Kant thought that we can know some general facts about the external world, i.e., that it exists, and that is governed by various laws of nature. Hume did not even allow this much. On the other hand, Hume devoted much of his attention to lesser grades of cognition—grades of cognition that present-day philosophers are willing to count as knowledge.
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