Critical thinking is the disciplined process of analyzing, evaluating, and reasoning about information to form well-justified beliefs and decisions. As a practice, it rests on logic, which is the study of valid inference and the principles that distinguish correct reasoning from incorrect. When we engage in critical thinking, we typically work with arguments—sets of statements intended to support a conclusion. The statements that provide support are called premises, and the statement that the argument aims to establish is the conclusion. Recognizing the structure of an argument is the first step toward evaluating it.
The strength of an argument is judged along two dimensions. A valid argument is one whose conclusion must be true if its premises are true; the conclusion follows necessarily from the premises. An invalid argument is one in which the conclusion could be false even when the premises are true. Validity concerns logical structure, not factual content. A sound argument goes further: it is a deductively valid argument with all true premises. Thus soundness combines validity with truth, while validity alone guarantees only that truth is transmitted, not that the premises actually are true.
Reasoning itself takes several forms. Deductive reasoning moves from general premises to a specific conclusion that follows with necessity—if the premises are true, the conclusion cannot be otherwise. Inductive reasoning works in the opposite direction, drawing a probable general conclusion from specific observations. Abductive reasoning, sometimes called inference to the best explanation, formulates the most plausible account of the available evidence. A classic deductive form is the syllogism, such as: all humans are mortal; Socrates is human; therefore, Socrates is mortal.
Two important patterns of deductive inference are modus ponens (if \(P\) then \(Q\); \(P\); therefore \(Q\)) and modus tollens (if \(P\) then \(Q\); not \(Q\); therefore not \(P\)). These forms are valid because the conclusion necessarily follows from the premises whenever logical entailment holds—whenever the truth of one statement requires the truth of another. Related to entailment are the notions of necessary and sufficient conditions: a necessary condition must be present for an outcome, while a sufficient condition, by its presence alone, guarantees that outcome.