Propositions and their truth values are two elemental ingredients of logical reasoning.
One type of thought process is an argument. An argument is a series of statements; some of these statements are premises: assertions, reasons, claims; from these premises is derived a conclusion. The argument claims that, because the premises are true, the conclusion is true. If the conclusion does indeed logically follow from the premises, the argument is valid; if the conclusion does not logically follow from the premises, the argument is invalid. Note that the words “valid” and “invalid” apply to conclusions or to
arguments, not to premises. When we refer to premises, we describe them as being true or untrue. Whenever we want to evaluate an argument, we
should examine both the premises and conclusions. The premises, i.e., the evidence, should be thorough and accurate; the conclusion should clearly and incontrovertibly derive from that evidence. When an argument
is unsuccessful, it has probably gone wrong in one of the following areas:
1. The evidence has not been thorough; contradictory evidence has been overlooked or ignored.
2. The evidence has not been accurate; false or unsubstantiated or misleading statements have been claimed as fact.
3. The conclusion has not clearly and uncontrovertibly come from the evidence; the relationship between evidence and conclusion has not been a firm one.
When one or more of these phenomena occur in an argument, that argument is said to be fallacious. The argument claims to have done something that it in fact has not done. Another way to describe arguments is to determine whether they are sound or unsound. To be sound (1) the premises must be true and (2) the conclusions must logically follow from these premises. If either of these conditions is violated, the argument is unsound.Robert J. Gula, Nonsense: Red Herrings, Straw Men and Sacred Cows: How We Abuse Logic in Our Everyday Language (2007).