Lo, Quine! 
In Theories and Things and Perspectives on Quine, Quine defines an observation sentence for an individual in this way:
If querying the sentence elicits assent from the given speaker on one occasion, it will elicit assent likewise on any occasion when the same total set of receptors is triggered; and similarly for dissent. [PoQ, p.3]
Yet this seems obviously inadequate. Consider and individual asked to watch a series of images and count the number of rabbits that appear. Obviously, "That is the first rabbit" ought to count as an observation. Nevertheless, the very same image and so the very same receptor trigger will not elicit assent on subsequent occasions. Rather, when the subject sees the same picture she will say "That is the second rabbit."

One might amend the definition so as to include context; for example, the agent will assent when the same receptors are triggered and the neural machinery responsible for memory is active in the same way. This seems inadequate for several reasons. First, the revised version individuates occasions so specifically that they might be unrepeatable. This would trivialize the consequent of the conditional. Second, the revised version makes remembering something count as an observation. Third, there may be no clear line between the neural machinery of memory and that of calculation. That would make arithmetic count as observation.

Similar problems arise for sentences like "It is cold." My sensitivity to cold depends in part on my state of mind and attention. Assent requires more than just a specific temperature on my receptors.

In addition to the specific objection, I disagree with Quine's general fixation on stimulations at the sensory periphery. Perception and memory are richly interwoven with the environment; cf. d-cog.

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