Arguments of this type are less airtight than valid ones, but they are extremely important, since you often need to make some judgment when the information that you have at your disposal is not enough for a valid argument, but is enough to make some alternative likelier than others. An argument that occurs in a text is successful as a likely argument if it does three things. The premises of the argument are among the things being taken for granted. The conclusion of the argument should be the point that is at issue. Each element of an argument that is presented as following from some other claims should be likely if those claims are true.
Likely arguments can be better or worse, depending on how likely the premises make the conclusion.
The theory of logic that deals with probable inference is not nearly so well understood.