In this video, Geoff Pynn (Northern Illinois University) follows up on his introduction to critical thinking by exploring how deductive arguments give us reason to believe their conclusions. Good deductive arguments guarantee their conclusions, and so must be valid (i.e., it must be impossible for the premises to be true while the conclusion is false) and have true premises. Philosophers call arguments like these "sound". You can see whether an argument is sound by trying to think of a counterexample to it, but to see whether its premises are true, you need to do some research.
Northern Illinois University