[T]he theory of meaning is the fundamental part of philosophy which underlies all others. Because philosophy has, as its first if not its only task, the analysis of meanings, and because, the deeper such analysis goes, the more it is dependent upon a correct general account of meaning, a model for what the understanding of an expression consists in, the theory of meaning, which is the search for such a model, is the foundation of all philosophy, and not epistemology as Descartes misled us into believing.

Some writers have put forth the view that whenever one knows, one knows that one knows. There is an immediate stumbling block to this, however. One may know yet not believe one knows; with no existing belief that one knows to do the tracking of the fact that one knows, one certainly does not know that one knows.

One has animal knowledge about one’s environment, one’s past, and one’s own experience if one’s judgements and beliefs about these are direct responses to their impact – e.g., through perception or memory – with little or no benefit of reflection or understanding.

One has reflective knowledge if one’s judgment or belief manifests not only such direct response to the fact known but also understanding of its place in a wider whole that includes one’s belief and knowledge of it and how these come about.

Criteria are ‘criteria for something’s being so’, not in the sense that they tell us of a thing’s existence, but of something like its identity, not its being so, but of its being so. Criteria do not determine the certainty of statements, but the application of the concepts employed in statements.

The only facts which seem genuinely independent of any scientific theory are those of the present experiences of touch, taste, smell, hearing and sight that each individual scientist is currently experiencing. But such facts are not, of course, public facts, they are private to each individual. So we have the dilemma that, if facts are truly independent of theory they are private and do not form part of the public domain of knowledge; if they are public facts they are affected by all sorts of influences particularly from previous knowledge and upon which their exact form and our confidence in them depend. At least for science, there are no brute facts.

On the path of objective knowledge […] we shall […] remain on the outside of things; we shall never be able to penetrate into their inner nature, and investigate what they are in themselves. […] So far I agree with Kant. But now, as a counterpoise to this truth I have stressed that […] we ourselves are the thing in itself. Consequently, a way from within stands open to us to that real inner nature of things to which we cannot penetrate from without. It is, so to speak, a subterranean passage, a secret alliance, which, as if by treachery, places us all at once in the fortress that could not be taken by attack from without. Precisely as such, the thing in itself can come into consciousness only quite directly by itself being conscious of itself […].

Knowledge requires coherence, true enough, but it often requires more: e.g., that one be adequately related, causally or counterfactually, to the objects of one’s knowledge, to one’s environment or surroundings, which is not necessarily ensured by the mere coherence of one’s beliefs, no matter how comprehensively coherent they may be. […] Knowledge requires not only internal justification or coherence or rationality, but also external warrant or aptness. We must be both in good internal order and in appropriate external relation to our surrounding world.