A more recent text-book definition of critical thinking by Michael Scriven and Richard Paul, puts it this way: "Critical thinking is the intellectually disciplined process of actively and skillfully conceptualizing, applying, analyzing, synthesizing, and/or evaluating information gathered from, or generated by, observation, experience, reflection, reasoning, or communication, as a guide to belief and action. In its exemplary form, it is based on universal intellectual values that transcend subject matter divisions: clarity, accuracy, precision, consistency, relevance, sound evidence, good reasons, depth, breadth, and fairness." While all these descriptives aptly illuminate the skill, the process of critical thinking has been a long time in the making.
Inspired by the early Greek tradition of critical thinking, the middle ages evolved a systematic mode of thinking premised on well-reasoned foundations that could be cross-examined at will. Renaissance scholars, such as Francis Bacon in England, extended this mode of inquiry by emphasizing the value of empirical reasoning that can be "tested" against the occurrence of natural phenomena in the world. In fact, as criticalthinking.org duly notes, Bacon's focus on inductive methodology (or reasoning from particular to general in drawing conclusions) reflected the new paradigm that would eventually be adopted by modern science.
The Copernican Revolution of the seventeenth century was one major consequence of this development in critical thinking. By analyzing evidence gathered from the natural world rather than issued by fiat from the Catholic Church, thinkers such as Galileo and Kepler conclusively demonstrated that the Earth-centered theory of the universe was fundamentally misconceived.
In the social and political realm, this same distrust of orthodoxy fueled the French Enlightenment. Major breakthroughs in a variety of disciplines--from Adam Smith, Kant, and Spencer, to Karl Marx, Darwin and Freud--similarly stem from the belief that intellectual prejudices in all forms must be subjected to rigorous, critical examination.
In essence, critical reasoning is based on your ability to differentiate strong arguments from weak ones. If an argument is based on a set of claims (or premises) supported by well-reasoned facts, then the conclusion will, more likely than not, deserve merit. Alternatively, if that same argument teeters on a faulty premise or questionable assumption, then its basis will seem skewed. Remember that the move from premise to conclusion requires an inference. So, for instance, if we assume that a fetus is a human being, and that the killing of innocent human beings is murder, then we could "reasonably" conclude with the inference that abortion "is" murder. Opponents, however, would be quick to challenge the validity of the first assumption--"Who" says a fetus is a human being?
One way to determine an argument's validity is to look at its form. As Conway and Munson observe in "The Elements of Reasoning," "modus ponens" and "modus tollens" represent two of the more common rules of inference in critical thinking. Consider, for instance, this example of the modus ponens form: "If the spider is a brown recluse, then it is dangerous. The spider is a brown recluse. Therefore, it is dangerous." Note that the form of this argument (If P, then Q; given P; Therefore, Q) is valid even if the two premises turn out to be false. This means that the reasoning is sound regardless of the truth or falsity of the argument's claims. The form of modus tollens, in contrast, denies the consequent affirmed in the conditional. So rather than affirming the second premise in the spider example ("The spider is a brown recluse"), a modus tollens argument would negate this assertion ("The spider is not brown"), and as a result negate the conclusion: "Therefore, it is not dangerous."
The ability to detect logical fallacies notwithstanding, drawing attention to the various ways we form arguments and substantiate claims ultimately leads to a more honest and informed understanding of the beliefs we debate and the ideas we value.