>> Kleene's that have three truth values (i.e. values assigned to literals): true, false ...and unknown. ... which is to say, "null". Regardless of what logic

>> Kleene's that have three truth values (i.e. values assigned to literals): true, false ...and unknown. ... which is to say, "null". Regardless of what logics exist the statement you make - that "unknown" is "null" - is actually wrong and the heart of the problem. "Unknown" is only one semantic interpretation of null; there are many others, and problems arise in software development because of those (sometimes slight) differences in the interpretation of what "null" should reflect in the real world. Some sources suggest as many as 129 possible semantic meanings for "null", which is why Date rails against "null" values in SQL. With Option/Maybe we're constrained to the universe of values of that type plus exactly one more value (None); with null who knows how many of the 129 possible meanings of the null value we are dealing with in addition to the base type? Null is computationally and mentally expensive to deal with.