Quine on observation sentences

One such step, which is emphasized in Quine’s later work, is the mastery of what he calls observation categoricals. These are sentences of the form “Whenever X happens, Y happens”, where the variables are to be replaced by observation sentences. (E.g. “Whenever there’s smoke, there’s fire.”) It is plausible to suppose that a child who has learnt the observation sentences can come by a mastery of the relevant observation categorical. The observation sentences are what Quine calls occasion sentences, true on some occasions and false on others, whereas the observation categoricals are eternal sentences, true or false once for all. Quine suggests that we can think of observation categoricals as a plausible first step into mastering eternal sentences, which make up our serious theoretical knowledge.