An argument is a set of one or more propositions (or claims), called premises , advanced as proof or evidence for another proposition, called the conclusion.

A cogent argument must meet three conditions. If any the conditions is not met, the argument is fallacious:

(a) the reasoner(s) must be warranted in believing the premises;

(b) the argument must proceed according to a correct principle of reasoning;

(c) one must argue in good faith and not suppress relevant evidence.

If these conditions are met, one is warranted, or justified, in believing the argument's conclusion.

If these three conditions are met, one is warranted, or justified, in believing the argument's conclusion.

N.B. Argument layeredness.

In fact, one should believe the conclusion. This is an epistemic valuation and use of "should" that could possibly be overridden by considerations of prudence or ethics (e.g., the cancer patient who should believe that she will recover despite the evidence, or the husband who should believe his wife is faithful despite the evidence).