Let us say that knowledge is "a set of true statements based on compelling reasons." (This is the same as the familiar definition of knowledge as "justified true belief".) Philosophers offer a variety of claims, and they offer arguments for their positions. But do they offer knowledge about anything? Is it possible to say that "Philosophical statement P is true", or only that "Philosophical statement P is supported by strong and compelling reasons"? Are philosophical topics within the domain of things concerning which we can have knowledge at all?
We know what kind of knowledge empirical science provides: factual knowledge about the world and inferences or theories supported by empirical observation. We also know what kind of knowledge is offered by mathematics and logic : deductive knowledge derived from a set of axioms in one or another of the fields of mathematics. And we can specify the nature of the knowledge provided by linguistics and semantics: expository knowledge of the meanings of various words and phrases in ordinary usage. Putting these areas of inquiry very crudely, we might summarize them as "inductive knowledge," "deductive knowledge," and "semantic knowledge." But what does philosophy add to our knowledge and understanding of human experience and knowledge?
Notice that knowledge and truth are interrelated. Truth, in turn, has to do with correspondence and reference. Statements refer to things and properties; statements are true or false depending on whether the things to which the statement refers in fact possesses the properties attributed to it. So a statement cannot be a piece of knowledge if it does not permit of correspondence to some independent set of facts.
Now consider the kinds of reasoning and statements that occur within philosophy.
First, philosophy offers assertions based on rigorous analysis of concepts and conceptual relationships. This exercise looks a bit like the linguistics/ semantics option above; but philosophers offer "value-added" by providing rational reconstruction of the concepts they consider. They reconstruct and improve upon the concepts of everyday language.
Second, philosophy may provide constructive analysis and further development of the methods and conceptual systems of various disciplines, including the empirical sciences. Here the philosopher is purporting to offer substantial findings that will improve upon the epistemic characteristics of existing scientific practices. Crudely, a deeper understanding of the logic of evolutionary explanation and the mathematics of natural selection is a philosophical effort, and it is a substantive contribution to the philosophy of biology and to biological theory. Here, we must ask whether there is a credible basis for the contribution. In what respect does the philosopher possess a distinctive basis for providing rational assessment and recommendations about the structure and practice of empirical science? Typically, philosophers would respond by saying that philosophical argumentation about the methods of science gains credibility through the understanding of rationality and reasoning that philosophers possess.
Third, philosophy can construct ethical theories that possess some degree of justification, beyond ordinary moral opinions. When John Rawls asks, "What is justice?", he begins with some ordinary associations with the concept, but then builds out a theory of justice that goes far beyond ordinary language. And he offers a range of philosophical justifications in support of his theory of "justice as fairness".
A major problem in answering the key question, what is the nature of philosophical knowledge, is the fact that we ordinarily divide knowledge into "empirical/contingent" and "formal/necessary" (or what Quine calls the "analytic/synthetic" distinction). But philosophers seem to advance their theories in a way that implies they are neither empirical nor purely formal. Kant puts this point in the form of a category of knowledge that is "synthetic a priori" -- that is, knowledge that is based on purely philosophical reasoning but that is nonetheless not purely formal. (Kant's claim that the statement "reality is spatially organized in Euclidean three-dimensional space" is true synthetic a priori asserts that the statement is necessarily true but is not a truth of logic.)
One possible stance that we might take is to restrict the concept of "knowledge" to statements about empirical states of affairs (where truth and falsity are possible), but to then postulate other categories of belief that have a degree of rational credibility without correspondence to the empirical world. On this approach, empirical statements, suitably supported, can constitute knowledge of the empirical world. Philosophical statements -- statements about scientific method, the nature of beauty, or ethical principles -- can be offered with rational justification but do not have the referential structure that would permit them to be "true" and do not count as knowledge. If we take this approach, then we may be justified in accepting the proposition "Justice is fairness," but we would not be justified in thinking it is "true." And therefore the statement is not an example of "knowledge".
This approach sounds a bit parallel to the position of early twentieth-century logical positivism, which maintained the verificationist theory of meaning -- the meaning of a sentence is the set of conditions that establish its truth or falsity -- with the result that theology, philosophy, or ethics are strictly speaking meaningless. However, the two views are not the same, because this view permits that philosophical discourse is meaningful and rational; it simply denies that the statements that philosophers make either correspond or fail to correspond with facts about the world.
The conclusion to this line of thought is somewhat startling: philosophy does not offer knowledge at all. However, it does provide opinions and statements that are founded on good reasons, and we have rational grounds for believing these statements.