Given a model of appropriate vocabulary, a sentence such as 
is either true or false in that model. To put it more formally, there is a relation called truth which holds, or does not hold, between sentences and models of the same vocabulary. Now, how to verify if a given sentence is true in a given model is obvious in most cases (for example, in order to check the truth of we simply need to check if every individual in the model is a moron). What is not so clear is how to give a precise definition of this relation for arbitrary sentences. This is going to be the problem this section deals with.