Logic

Logic (from ancient Greek ????? (logos), meaning reason) is the study of 
arguments. Its primary task is to set up criteria for distinguishing good from 
bad arguments. Arguments express inferences—the processes whereby new assertions 
are produced from already established ones. As such, of particular concern in 
logic is the structure of arguments—the formal relations between the newly 
produced assertions and the previously established ones, where "formal" means 
that the relations are independent of the assertions themselves. Just as 
important is the investigation of validity of inference, including various 
possible definitions of validity and practical conditions for its 
determination. It is thus seen that logic plays an important role in 
epistemology in that it provides a mechanism for extension of knowledge.

As a byproduct, logic provides prescriptions for reasoning, that is, how 
people—as well as other intelligent beings, machines, and systems—ought 
to reason. However, such prescriptions are not essential to logic itself; 
rather, they are an application. How people actually reason is usually 
studied in other fields, including cognitive psychology.

Traditionally, logic is studied as a branch of philosophy. Since the 
mid-1800s logic has been commonly studied in mathematics, and, even more 
recently, in computer science. As a science, logic investigates and 
classifies the structure of statements and arguments and devises schemata 
by which these are codified. The scope of logic can therefore be very large, 
including reasoning about probability and causality. Also studied in logic 
are the structure of fallacious arguments and paradoxes. The ancient Greeks 
divided dialectic into logic and rhetoric. Rhetoric, concerned with 
persuasive arguments, would currently be seen as contrasted with logic, 
in some sense; as is dialectic in most of its acquired meanings.