Cogency
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An argument is cogent if and only if the truth of the argument's premises would render the truth of the conclusion probable (i.e., the argument is strong), and the argument's premises are, in fact, true. Cogency can be considered inductive logic's analogue to deductive logic's "soundness." As an example, consider the following.
- Without looking, Jill pulled out 100 marbles from a bag; 95 of the marbles Jill pulled out were red.
- Therefore, the next marble Jill pulls out from the bag will be red.
The truth of the premises would, indeed, make the conclusion probable. Therefore, this argument is strong. If the premises are, in fact, true, then the argument is also cogent.
"Probable"
There is no standard of how likely an event must be for it to be called "probable." Just as there are degrees of probability, one can also speak of degrees of cogency. The degree of an argument's cogency, then, is a function of the argument's strength. In the above example, Jill pulling out a 101st marble that turns out to be red would make the conclusion even more likely, and therefore the argument stronger. Note that this feature of cogency is a disanalogy from deductive logic's "validity," since a deductive argument can be either valid or invalid and nothing in between.
Good argument
Good argument, as used by philosophers and many others, means simply a sound or cogent argument. If one has offered a sound or cogent argument in defense of one's conclusion, then one has stated a true view, or at least a probably true view. The premises of one's argument support, or, with some sophisticated complications aside, justify one's belief in the conclusion.
A good argument is the closest thing we have to a guarantee that a belief is true. If one is armed with a good argument, one has helped to justify one's belief in the conclusion, and to remove doubts about it.