We are strongly intuitive about the validity of various inferences. This may or may not be hard-wired into us. Intuition gets us into trouble sometimes. ‘Truth functions—or not?’ plays around with the idea of invalid and valid inferences and, using phrases such as ‘or’ or ‘it is not the case that’, shows that chaining valid inferences together doesn’t necessarily result in an invalid inference. People’s intuition can be misleading. Sentences can be true or false. Gottlob Frege called these truth values. If we assume that every sentence is either true or false, but not both, we can predict the conditions in what logicians call a truth table.