Valid Arguments

How can we criticize a valid argument?
A sound argument has true premise and its form is valid. We know the form is valid: so the only possible criticism of this argument is to claim that either premise 1 or premise 2 is false. The likely candidate is premise 2. One could argue that in open marriages one does not promise not to have affairs. Thus in open marriages one does not break a promise when one has an affair, and the act of having an affair (called adultery in this context) is not immoral.

Truth and Validity
There is a connection between truth and validity. If a valid argument has true premises, then the conclusion of this valid argument must be true. A valid argument is an argument that can not have true premises and a false conclusion. Let us think about two different kind of true premises. Let us call one knowledge and the other true opinion and we will mean by true opinion not the opinion of the wise or the few who have studied the subject for a long time, but the majority of the audience the speaker or writer is addressing. True premises in this rhetorical sense are offered as a means to persuade a particular audience. The speaker can say to the audience: since you accept these premises, you must accept the conclusion that follows from the premises. The fact that the audience believes these premises are true is the important point from a rhetorical point of view (not logic): premises that are in fact false but are believed to be true by the audience will persuade them. It is also true that the conclusion that is believed to follow from some premises assumed to be true for a particular audience, but it fact does not, is also believed to be true for that particular audience.. Sometimes conclusions only appear to follow from other statements but in truth do not. We as human beings often have false beliefs including false beliefs about "what follows from what." Thus, we think an argument is valid and it is not. The purpose of the art or science of logic is to study "what follows from what."