A skeptical thesis is typically a claim that the beliefs in a certain range lack a certain status. In addition, then, to varying in their scope -- which specifies the range of beliefs being targeted -- skeptical theses, and the arguments used to establish them, also differ in their force -- which specifies precisely what lack the skeptic alleges befalls the targeted beliefs. Skepticism, then, isn't limited to pessimistic accounts of the extent of our knowledge; they can be views on which any of the designations discussed above in section 2 surprisingly fail to apply to a wide range of our beliefs.  Theories according to which surprisingly few, or perhaps none, of our beliefs are justified, or rational, or adequately supported by our evidence, or known with complete certainty, etc., are also examples of skepticism.
    [For more introductory material on skepticism, including a description of some common skeptical arguments, and many of the most influential types of response to skeptical problems, see my "Responding to Skepticism," available on-line here.]

4. Internalism and Externalism.

As we've already noted, epistemologists are interested in the matters of when (under what conditions) beliefs are justified and when subjects know what they believe.  There is an important division between two main types of accounts of these matters -- that between internalism and externalism.  According to the epistemic internalist, these matters depend primarily on factors internal to the believer's point of view and/or factors to which the believer has special access. Most internalists accept that the external matter of whether a belief is true is relevant to the issue of whether it constitutes knowledge, so on the issue of knowledge, internalism is usually the position that only or primarily internal factors are relevant to whether true beliefs constitute knowledge. The epistemic externalist, on the other hand, claims that issues of knowledge and/or justification depend exclusively or primarily on such factors as how the belief was caused or how reliable is the faculty or mechanism by which the subject came to hold the belief -- matters which are not in the requisite way "internal" to the subject's point of view, as can be seen by the fact that you can imagine two subjects whose mental lives are identical with respect to how things seem to them from their own point of view, but whose beliefs diverge with respect to the matters in question. The internalist about justification will have to hold that the beliefs of such subjects have the same justificatory status (they're either both justified or both unjustified, and to the same degree), and the internalist about knowledge will have to hold that, so long as the beliefs of such "twins" are true in both cases, they can't diverge on the matter of whether they constitute knowledge.

So, consider an "internal twin" of me.  This twin's life was identical to mine up to midnight last night.  At that time, our life histories drastically diverge, but not in any way causes a difference in what our experiences seem like from the inside: Our "internal" lives are still identical.  At midnight, super-advanced aliens snatched my twin's brain from his body, placed it in a (human)-brain-sustaining vat, and hooked it up to a super-advanced computer, that, taking into account the output of the brain that is my twin, gives it appropriate sensory input.  Meanwhile, we may suppose, I remain a normally embodied human, with no aliens anywhere around me.  The aliens who snatched my twin's brain from his body are so advanced that they were able to do so in such a way that did not impact at all on his experience.  Now it is morning, and I have a conversation with my wife.  My twin is having identical experiences, and so thinks he is having a conversation with his wife, but in fact he is not.  (His wife is in fact now, unbeknownst to him, in shock and mourning over the discovery of his de-brained, dead body.)  The internalist about justification will hold that my belief that I am having a conversation with my wife has the same justificatory status as does my twin's analogous belief: either we are both justified in our belief or both unjustified, and to the same degree.  For what it's worth, the internalist has always seemed to me to be right about this: It seems to me that such twins can't differ from one another on the justificatory status of their beliefs: If my belief is justified, so is my twin's; if his is unjustified, so is mine.  In the case under discussion, I think both me and my twin are justified in holding the belief in question -- even though my twin's belief is false.

What about knowledge?  Since my belief (that I'm having a conversation with my wife) is true, while my twin's belief is false, even internalists, at least as I construe them, can hold that one of us (presumably, me) knows the item in question, while the other (presumably, my twin) doesn't.  For a good test case, we need an example where the beliefs in question are both true.  So: I am holding a cup of coffee.  My twin also believes he is holding a cup of coffee, but in fact he isn't.  Because I (correctly) believe I am holding a cup of coffee, I believe that there is a cup of coffee within 10 feet of my brain.  (If you are not a fan of "implicit" beliefs, you may suppose that I have just been asked whether there is a cup of coffee within 10 feet of my brain, and so have considered the matter and have come to a positive conclusion, and, of course, then, that my twin has had experiences that make him think that he has just been asked whether there is a cup of coffee within 10 feet of his brain, and has come to a positive conclusion.)  So my twin also believes there is a cup of coffee within 10 feet of his brain.  He believes this because he (incorrectly) believes that he is holding a cup of coffee.  But while he is wrong about the matter of what he is holding, let us suppose that he turns out to be right about the fact that there is a cup of coffee within 10 feet of his brain: The aliens who have snatched his brain have taken up the human practice of drinking cups of coffee, and one of the aliens has carelessly left a cup of coffee resting right next to the vat that holds the brain of my internal twin.  So, as it happens, my twin's belief that there is a cup of coffee within 10 feet of his brain is true.  So here we have a pair of "twins" who share a certain belief that is true in both of their cases.  The internalist will say that either both me and my twin know that there is a cup of coffee within 10 feet of his brain, or that neither of us knows that.  Since it seems to me that I do know know that there is a cup of coffee within 10 feet of my brain, but that my twin doesn't know that of himself, the externalist seems to me to be right about knowledge.  Knowledge seems to me to crucially involve matters that go beyond true belief plus purely "internal" issues: there are "external" matters beyond the truth of the belief in question that matter to whether a belief is a piece of knowledge.  (Since I think my twin is, like me, justified in believing there is a cup of coffee within 10 feet of his brain, but does not know that this is true, even though it in fact is true, I think the case of my twin's belief that there is a cup of coffee within 10 feet of his brain is a "Gettier case," in one common use of that term: It is a case of a justified, true belief that is nevertheless not a piece of knowledge.)

5. The Structure of Knowledge: Foundationalism and Coherentism.

An important issue for epistemologists is over the structure of knowledge (or of justification).  The main positions on this issue, foundationalism and coherentism, are perhaps best introduced as reactions to the problem of the regress of reasons.  At least sometimes, a belief, A, constitutes a piece of knowledge or is justified because it is based on another belief, B, that one holds and that constitutes evidence for A.  But it seems this can only work if belief B is itself a piece of knowledge or is a justified belief.  But how did B get to be justified?  Perhaps it was based on still another of one's beliefs, C.  But, again, it seems this can only work if C is already justified or known.  Where and how can the process of basing beliefs on other beliefs come to an end?  If we demand of all of our beliefs that they be properly based on evidence in the form of other beliefs we hold in order to be justified, and if we demand that the lines of evidence contain no "circles" of justification (that it can never happen that A is justified because it is based on B, which is justified because it is based on C, and so on, until we reach a belief that is justified because it is based on A), and if we admit that these lines of evidence cannot be infinitely long, we will be led to the skeptical conclusion that none of our beliefs are justified (or are knowledge, if we run this problem on knowledge rather than justification).
    Foundationalists avoid this skepticism by denying that all of our beliefs need to be based on other of our beliefs in order to be justified.  According to the foundationalist, some of our beliefs are properly basic (to use a term that, as far as I know, originated with Alvin Plantinga): they are justified (or are knowledge) independent of their being based on any other beliefs.  These properly basic beliefs then serve as the "foundation" upon which all of the rest of our justified beliefs are "built": All of our justified beliefs that are not properly basic are based, directly or indirectly, upon this foundation of properly basic beliefs.  Foundationalism itself is just a commitment to this basic structural picture.  Versions of foundationalism will differ with one another over the matter of which of our beliefs can be properly basic (can be justified without being based on other of our beliefs), and over what constitutes a successful basing of one belief upon another.
    Coherentists hold that only evidence-like relations among one's beliefs can render any of our beliefs justified, and they thus reject the foundationalist's properly basic beliefs.  The coherentist instead avoids the regress argument by accepting that there can be "circles" of justification: sometimes A can be justified by being based on B, which is (perhaps indirectly, through a long series of basings) based on A.  What renders our beliefs justified on the coherentist picture is how well our beliefs cohere with one another, rather than on how well they are based on some foundation of "properly basic" beliefs.  Again, coherentism itself is just a commitment to this basic picture of the structure of our justified beliefs, and this basic picture gets worked out in a myriad of significantly different ways.
    I should stress that although it's handy to introduce these basic structural options as responses to the regress argument, advocates of these positions don't necessarily come to these positions just in order to avoid the regress argument.
    While, as I have stressed above, there are many significantly different versions of both foundationalism and coherentism, there are also views that fall into the gap between these two camps, and represent something of a compromise between them.  (On this, you can see my paper, "Direct Warrant Realism" [pdf, word], where, in sections 2-4, I defend the such a structural compromise position, and, in section 1, I explain, and in sections 5-7 I defend a particular account of the justification of perceptual beliefs -- "Direct Warrant Realism" -- that exemplifies this compromise structure.)

Other Topics

The above is just a sample of the kinds of topics treated by epistemologists.  Other epistemologists no doubt would have chosen different sets of topics to explain.  Still, the above quick discussion should serve to give you some idea of the type of issues treated in epistemology -- and there are some links to some encyclopedia articles on various other topics below.  If you want to dig more deeply, one good place to go next is one of the anthologies listed toward the bottom of section 2 of The Epistemology Page.  See the different topics into which essays are organized, in, for instance, the Sosa & Kim, ed. anthology, Epistemology, and then you will also have at hand some of the more important essays on the topics that interest you.

If others inform me of good brief introductory explanations of important topics in epistemology that are available on-line, I will post links to them here.

See Also These Introductory On-Line Articles:

On Epistemology in general:
Joseph Cruz's Nature Encyclopedia of Cognitive Science article, "Epistemology" (pdf document)
Peter Klein's Routledge Encyclopedia of Philosophy article, "Epistemology".

On Particular topics in epistemology:
Elizabeth Anderson's Stanford Encyclopedia of Philosophy article, "Feminist Epistemology and Philosophy of Science".
Tim Black's Internet Encyclopedia of Philosophy article, "Contextualism in Epistemology".
Michael Brady and William Harms's Stanford Encyclopedia of Philosophy article, "Evolutionary Epistemology".
Keith DeRose's editor's introduction to Skepticism: A Contemporary Reader, "Responding to Skepticism".
Richard Feldman's Stanford Encyclopedia of Philosophy article, "Naturalized Epistemology".
Peter Forrest's Stanford Encyclopedia of Philosophy article, "The Epistemology of Religion".
Richard Fumerton's Stanford Encyclopedia of Philosophy article, "Foundationalist Theories of Epistemic Justification".
Alvin Goldman's Stanford Encyclopedia of Philosophy article, "Social Epistemology".
John Greco's Stanford Encyclopedia of Philosophy article, "Virtue Epistemology".
Jonathan Kvanvig's Stanford Encyclopedia of Philosophy article, "Coherentist Theories of Justification".
George Pappas's Stanford Encyclopedia of Philosophy article, "Internalist vs. Externalist Conceptions of Epistemic Justification".
Matthias Steup's Stanford Encyclopedia of Philosophy article, "The Analysis of Knowledge".

For a defense of what he calls "veritistic epistemology" ["Veristic epistemology (whether individual or social) is concerned with the production of knowledge, where knowledge is here understood in the 'weak' sense of true belief.  More precisely, it is concerned with both knowledge and its contraries: error (false belief) and ignorance (the absence of true belief).  The main question for veritistic epistemology is: Which practices have a comparatively favorable impact on knowledge as contrasted with error and ignorance?" (p. 5)] against "such views as social constructivism, postmodernism, pragmatism, cultural studies, and critical legal studies" (p. 7), see:
-Alvin Goldman's "Epistemology and Postmodern Resistance" (pdf document; note that there are a couple of blank pages at the beginning of the document), which is the first chapter of Goldman's Knowledge in a Social World (Oxford University Press, 1999).

For my best attempt to say what postmodernism is, and why I tend not to like it, with lots of links to material I've found helpful, see my blog post, "Characterizing a FogbankWhat Is Postmodernism and Why Do I Take Such a Dim View of It?".

Keith DeRose - Last modified 23 November 2005