  Quine considers acts of assenting to sentences (or dissenting; but we shall mostly leave that as understood). He focuses, in particular, on our dispositions to assent to sentences. (We will briefly consider the idea of a disposition in the next section.) To be an observation sentence, a sentence must fulfill two criteria, one individualistic and one social. The individualistic criterion is that a sentence is an observation sentence for a given person if he or she is disposed to assent to it when, and only when, he or she is undergoing appropriate sensory stimulations, regardless of her internal state (e.g. the ancillary information which he or she possesses). “It’s warm in here” presumably satisfies this criterion; my willingness to assent to it perhaps depends only on which of my sensory nerves are stimulated at the given moment.  “There’s milk in the refrigerator” presumably does not; unless I am actually looking into the refrigerator at the time, my willingness to assent to it depends not on what I am experiencing at the time but on my internal state, what I remember. The social criterion is that the individualistic criterion should hold across the linguistic community as a whole. To specify this more precisely is tricky, and we shall postpone the matter for a few paragraphs.