Active, persistent and careful examination of a belief in the light of the reasons that support it and the further conclusions it leads to — thinking that meets standards of rationality.

John Dewey (the American philosopher, writing in 1910) gave the classic definition: critical thinking is the active, persistent and careful scrutiny of a belief — or a supposed piece of knowledge — examined two ways:

• backwards, in the light of the grounds that support it ("why should I believe this?"), and
• forwards, in the light of the further conclusions it leads to ("what follows if I accept it?").

The word "critical" here means it answers to certain principles, norms or standards of rationality. That is precisely what separates it from irrational thinking, which fails to meet those standards — or meets them only weakly. So critical thinking isn't a mood or a personality; it's thinking held accountable to explicit quality criteria.

Tip: Note the two directions — grounds behind and consequences ahead. A belief that has no support behind it, or that you've never followed forward to see what it commits you to, hasn't been thought through.