Is knowledge possible?

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Is knowledge possible?
Domain\Thesis Declarative knowledge is impossible We have some declarative knowledge
Mathematics Mathematics is unrelated to "knowledge" We have proven knowledge in mathematics
Empiricism Empirical science is unrelated to "knowledge" We have justified knowledge in empirical science
Rejecting Theory We can't even "know" to reject a theory The theory-laden character of empirical observation is not fatal
Scepticism as Knowledge Sceptics can be sceptical of scepticism Scepticism is self refuting
Certainty Certainty is an absolute and can't be ranked Justification of our best knowledge is bigger then justification of sceptical premises
Self Awareness We don't "know" when we are standing up Reverting the sceptical claim
Solipsism Social consensus does not relate to "knowledge" Semantic externalist argument
Entailments Without "knowledge", there is no entailed knowledge Rejecting the Closure Principle



Moderator: Larry Sanger
Affirmative ("We have some declarative knowledge")
Lead summarist: Matthias Brendel
Summarists: List your name here if you want to join in on this side
Negative ("Declarative knowledge is impossible")
Lead summarist: Howard Burrows
Summarists: List your name here if you want to join in on this side
Copyeditors: List your name here if you want to edit spelling, grammar, etc. (leaving the sense of the text fully intact)
Jon Awbrey
NOTE. We welcome more participation in this summary! Right now we just have one person on each position and a moderator, which is the minimum required. See this section of the Debate Guide instructions about personnel roles. Note that we are committed to giving the "lead" positions, including the moderator position, to the best-qualified person who shows up.

Current instructions from the moderator

These instructions will change depending on where we are in the process.

This is the Debate Guide Project's first prototype of a summary, to help us think through and test proper format and procedure. Please see this outline of a proposed procedure.

  • The next step is: "On the wiki page, both (or all) sides first list out arguments in very summary form (one sentence per argument)." So please do that!
  • The next step after that will be: "The moderator oversees the matching of any arguments that directly respond to each other, that is, that argue for contrary claims, for example, 'the Iraq War increases the global terrorist threat' vs. 'the Iraq War decreases the global terrorist threat'. Such matching arguments, if any, are placed in a table of two or more columns."

I also have a request: If you know of (or are willing to find) a list about epistemology where you could introduce people to the DGP and this summary, that would be grand. I will post an announcement to PHILOSOP and PHILOS-L later, after our work is farther along.

I think we must have the following varieties of skepticism represented in a mature version of this page:

  1. Socratic skepticism about our ability to know the nature of things.
  2. Pyrrhonian global skepticism based on the diallelus/problem of the criterion.
  3. Cartesian solipsism (dreaming, doubt, evil geniuses, Putnamian brains in vats).
  4. Humean skepticism.
  5. Skepticism about other minds.
  6. Russell's skepticism about the existence of the world five minutes ago (or, about memory).

Then the idea is that, in a table just to the left of each of these skeptical arguments, there should be the leading sort of reply to that argument.

As to positive arguments for the possibility of knowledge, I would include:

  1. Arguments for a priori knowledge a la Plato, that brook no doubting of rational intuition.
  2. Descartes' argument that God is the guarantor of our knowledge.
  3. Reidian and Moorean commonsensism.
  4. Arguments that the concept of knowledge does not require absolute certainty.

How and whether these positive and negative arguments ought to be matched up remains to be seen.

Moderator's introduction

The debate about the possibility of knowledge is one of the very oldest, and one about which virtually every great philosopher has had something to say. "Is knowledge possible?" is a very broad question. Many books have been written for and against skepticism, and the reputations of at least three of the greatest philosophers — Socrates, Descartes, and Hume — rest largely on their formulations of, and attempts to overcome, skepticism, that is, the view that knowledge is impossible. So, like other summaries that concern very broad questions, this debate summary must be selective in how it interprets the question, what positions it entertains, and the number of arguments on each side. So this debate summary pits two positions, as follows.

On the affirmative side:

  • We have some declarative knowledge.

Notice that this is not in a "modal" form, that is, it does not say merely that knowledge is possible, but that we actually have it. Presumably, it is easier to argue that knowledge is possible than that we have it. The defender of knowledge is usually interested not in establishing the mere possibility of knowledge, but that we actually have it sometimes.

On the negative side:

  • Declarative knowledge is impossible.

Notice that this is in a "modal" form, that is, it says not just that we do not have knowledge, but that we cannot have it. The name for this general position — there are many varieties — is skepticism. According to this view, there is nothing that human beings can do to establish that they do have knowledge. Typically, the skeptic raises some grounds for doubt that apply to very large classes of belief, for example, all belief based on sense perception, and then the skeptic claims that we cannot have knowledge unless we remove those grounds for doubt.

Logically speaking, there is another position that we are not considering here, namely, that while declarative knowledge is possible, we do not actually have it.

Notice that both sides are deliberately unclear about what kind of knowledge is to be considered. There have been relatively few thoroughgoing skeptics, who denied that any knowledge is possible. Socrates was skeptical of our ability to know the true account or logos of things. Descartes was, at least initially in the Meditations, skeptical mainly of knowledge gained by sense-perception — though he did at least give lip service to doubts about mathematics. Hume's skepticism was pretty thoroughgoing but he is most famous for his skepticism about our ability to know causal relations and the external world distinct from our perceptions. So epistemologists, namely, those who study the nature and possibility of knowledge, often speak of skepticism about this or that.

What follow, then, are some of the most famous skeptical arguments and some of the most famous replies to them. More specialized debate summaries may follow that address the merits of more specific types of defenses of knowledge, or of skepticism.

Arguments for Thesis 0: "Declarative knowledge is impossible"

Claim 1. Mathematics is unrelated to "knowledge".

All mathematics provides is yet another "model" to compare by analogy. Knowledge entails how you justify the use of a model, whether it uses mathematics or some other structured analogy.

Claim 2. Empirical science is unrelated to "knowledge".

One can only observe what is already presumed to exist by current theory. "Data" are not "given" — rather what counts as "data" are observations excerpted out of context to defend a theory. Reason is applied to sensor input to rationalize away as "bad data" that which does not conform to current theory.

Arguments for Thesis 1: "We have some declarative knowledge"

Claim 1. We have proven knowledge in mathematics

In mathematics we have true and proven statements. True and proven statements deserve to be described as knowledge.

This explanation for later. For mathematical statements we have well established ways of proving a thesis based on an axiom-system and the rules of classical logic. Proven statements in mathematics are true, supposing that there is no human error in the proof. But since proving is made so formal, that this is highly unlikely.

Claim 2. We have justified knowledge in empirical sciences

In empirical and especially in natural sciences we have some justified theories, which we accept as true because of this. True, justified theories, which we accept as true based on the justification deserve to be denoted as knowledge.

A main sceptical argument is based on the thesis that we never can prove any empirical statement. This is true, but this requirement is not needed.

There is a debate on the meaning of the word "knowledge" in the empirical area. There appears to be a wide consensus that the necessary condition for saying that S knows p, where S is a person and p is a proposition, is the all of of the following are true:

  1. S believes p. That is, S accepts p to be true.
  2. p is true.
  3. p is justified.

Morever, there is a consensus that these three criteria are not sufficient. The so called Gettier problem shows that there must be other criteria, that link the justification to the truth of p. See (Steup 2006).

There is a debate on the precise nature of the extra criteria. But whatever possibilities are debated for this criteria, they seem to be possible to be fulfilled for some of our opinions.

Claim 3. The theory-laden character of empirical observation is not fatal

It is widelly accepted that empirical observation is theory-laden. A well known description of this is in Thomas Kuhn's book The Structure of Scientific Revolutions. However, few claim that this implies no genuine influence from observations. There is no widelly accepted claim that the theory-ladenness of empirical data would inhibit the acquisition of knowledge. In the history of science there are several cases where observations serve to reject a theory — if not in a simple falsificationist way, then at least in the sense that the old theory gradually degenerates over time. Franklin et al. and Brown showed that in several cases theory-laden observation may serve to test a theory against another theory. See (Brown 1993).

Claim 4. Scepticism is self refuting

Any of the sceptics argument against knowledge can be applied to scepticism itself. If knowledge is impossible, because we err, so the sceptic arguments may also be mistaken. If knowledge is impossible then the sceptics point of view is also not knowledge, i.e. the sceptic can not know what he claims. See (DeRose 1999).

Claim 5. Reliability of our best knowledge is bigger then reliability of sceptical premises

One of Moore's arguments is basically that we can not be certain about the most reliable knowledge we have, but the certainty of these knowledge is greater than the certainty of any sceptical premise.

"And what I can't help asking myself is this: Is it, in fact, as certain that all four of these assumptions are true, as that I do know that this is a pencil or that you are conscious? I cannot help answering: It seems to me more certain that I do know that this is a pencil and that you are conscious, than that any single one of these four assumptions are true, let alone all four. That is to say, though, as I have said, I agree with Russell that (1), (2), and (3) are true; yet of no one even of these three do I feel as certain as that I do know for certain that this is a pencil. Nay more: I do not think it is rational to be as certain of any one of these four propositions, as of the proposition that I do know that this is a pencil." -- Moore 1959, p. 226

Claim 6. Reverting the sceptical claim

Another argument of Moore reverts the direction of the sceptical arguments. And states that an argument in the other direction seems to be also plausible.

"I can therefore just as well argue: since I do know that I'm standing up, it follows that I do know that I'm not dreaming; as my opponent can argue: since you don't know that you're not dreaming, it follows that you don't know that you're standing up. The one argument is just as good as the other, unless my opponent can give better reasons for asserting that I don't know that I'm not dreaming, than I can give for asserting that I do know that I'm standing up." -- Moore 1959, p. 247

Claim 7. Semantic externalist argument

Putnam argues that if we were brains in the vat (this is the modern version of the sceptical dreaming argument), then we could not think of real things, like trees if we would not be in causal contact with real trees. Brains in the vat would not think of the same thing when they think of a tree. "Tree" in vat language means something different than in real language. See (Brueckner 2004).

Claim 8. Rejecting the deductive closure principle

The deductive closure principle (DCP) can be given the following intuitive formulation:

  • If you know that the proposition P to be true, and you know that P entails the proposition Q, then you also know that the proposition Q is true.

The sceptical objection can be summed up roughly as follows:

  • Since you do not know that the agent's hypothesis, H, is false, and since, given deductive closure, you would be able to know that the hypothesis is false if you knew O, the proposition that you would ordinarily think you know, for example, that you are aware, you must not know that O is true.

Nozik rejects the DCP by the following reasoning. For Nozik, knowledge that P is true has to fullfil all of the the following conditions:

  • P is true.
  • S believes that P is true.
  • If P were not true, then S would not believe that P is true.
  • If P were true, then S would believe that P were true.

Nozik claims that O fullfils them but H obviously does not. This means that H cannot be knowledge, but more importantly that the DCP is not true. The DCP, however, is a premiss of the sceptical objection. See (Luper 2006).


  • Brown, Harold I. (1993), "Discussion: A Theory Laden Observation Can Test a Theory", British Journal of the Philosophy of Science 44, 555–559. PDF.
  • Brueckner, Tony (2004), "Brains in a Vat", in Edward N. Zalta (ed.), Stanford Encyclopedia of Philosophy (Winter 2004 Edition). Eprint.
  • DeRose, Keith (1999), "Responding to Skepticism", in Keith DeRose and Ted A. Warfield (eds., 1999), Introduction to Skepticism: A Contemporary Reader, Oxford University Press, Oxford, UK. Online preprint with corrections.
  • Luper, Steven (2006), "The Epistemic Closure Principle", in Edward N. Zalta (ed.), Stanford Encyclopedia of Philosophy (Spring 2006 Edition). Eprint.
  • Moore, G.E. (1959), Philosophical Papers, George Allen and Unwin, London, UK.
  • Steup, Matthias (2006), "The Analysis of Knowledge", in Edward N. Zalta (ed.), Stanford Encyclopedia of Philosophy (Spring 2006 Edition). Eprint.


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