Modals are expressed by words such as "must", "have to", "ought to", "may", "can", "could", "should", "necessarily", "possibly". As a general rule, something is said to be possible if it is not necessary that it is not the case, and conversely, something is necessary if it is not possible that it is not the case, but beyond this, there is a variety of use. Here are a few distinctions between various kinds of possibilities. I will discuss in the next articles some criteria for disinguishing them, and attempt to reach a taxonomy. A logical or conceptual possibility anything that is not self-contradictory, that makes sense, that one can express or conceive without reaching a logical contradiction (for example respecting the fact that for any proposition, it must be that either p or not p"). Example: it is conceivable that aliens exist. An epistemic possibility anything that is com...