  Central to Quine’s naturalistic account of knowledge is the idea that all our knowledge is in some way based upon stimulations of our sensory nerves. For much of our knowledge, the relation is quite indirect. (This is one way of expressing holism; see 3.1, above.) Most sentences are not accepted because of a direct relation between the given sentence and stimulations of nerve endings; the connection goes via other sentences, and may be quite indirect and remote. But then there must presumably be some sentences which are directly related to stimulations. This is the role that observation sentences play in Quine’s thought. Acts of uttering such sentences, or of assenting to them when they are uttered by others, are shared responses to stimulation. (We shall enter some qualifications at the end of this sub-section.)