Semantically, function words are much closer to being able to be expressed as a logical function. For example, we can write a rule that gives a good approximation of the contribution of "and" to truth conditions in the following way: "A sentence of the form [S1 and S2] will be true only in the in case S1 is true and S2 is also true". Thus the sentence "It was raining and it was dark" will be true just in case the sentence " It was raining" is true and the sentence "It was dark" is also true.