  Quine returned to this problem on and off over the next thirty-five years. His eventual solution is that a sentence only counts as an observation sentence if an occasion which leads to my having neural intake which disposes me to assent to it also leads to your having neural intake which disposes you to assent to it. Here there is no cross-person identification of neural intake or cross-person standards of perceptual similarity. On this account, however, there can only be observation sentences if our standards of perceptual similarity line up in the right way. Two occasions which produce in me neural intakes which are perceptually similar by my standards must also (often enough) produce in you neural intakes which are perceptually similar by your standards. Quine is happy enough with this assumption of mutual attunement and suggests that it can be explained along evolutionary lines (see 1996, 160f.).