understanding others

Some philosophers (maybe Quine; Fodor seems to accuse him of this) hold that in order to understand what another is saying, you need to understand all the other's beliefs and concepts. For to understand what another is saying or believing, you need to understand what they mean, what their whole set of concepts and beliefs are, so you can accurately place their currently expressed belief in the whole package of beliefs and thus adequately understand it. This is a full or maybe even extreme holism. It seems a holism adapted to an overly idealized enlightenment conception (or misconception) of certainty.

I wonder if a moderate holism might allow that in order to understand another, understand reasonably well enough, you need to understand some of the other's closely related beliefs, but not the entire package of the other's beliefs (an impossible standard to achieve, thus leading to skepticism about ever understanding another). Example: if you say "I have a lot of reading to do for tomorrow, before class" I can assume we share similar beliefs about reading (what it involves, even if we disagree to some extent upon the level of comprehension needed to actually judge it as reading as opposed to simply skimming words) and the notion of a lot (even if there are relatively minor agreements about details) and the notion of class (I would be assuming, depending on who told me this, it would be a college class). But I do not need to assume beliefs about your theory of planetary or celestial motion (for the concept of "tomorrow"), or your beliefs about mental processes (for the concept of reading), or your beliefs about person identity (for the concept of I) in order to engage in sensible communication with you.

I worry that if you need to understand the entire package of another's beliefs in order to understand anything whatsoever that another asserts, understanding would be impossible. I also wonder if Derrida affirms the antecedent of my previous sentence, and so affirms also the consequent.