  How is Quine to explain the apparent necessity and a priori status of some truths without appeal to the Principle of Tolerance? Quine’s holism is the view that almost none of our knowledge is directly answerable to experience. (The exceptions are what he calls ‘observation sentences’; see 4.2, below.) In almost all cases the relation is indirect: a given sentence is only answerable to experience if a body of theory is presupposed. (When we say that a given observation confirms or refutes a given theoretical claim, we are tacitly presupposing other theoretical knowledge.) This is most easily seen if we consider a highly theoretical sentence, but in Quine’s view it holds, of almost all our sentences. The reason to accept a sentence is its contribution to the success of theory as a whole as an efficient and simple method of dealing with and predicting experience; in principle, this means the success of our theory as a whole, the whole body of sentences that we accept, in dealing with experience as a whole.