Assuming that the term critical thinking refers to a collection of mental processes that go into the critical evaluation of something, we may identify the following types of critical thinking on the basis of what is being evaluated. For instance, we may wish to evaluate:

1. the truth or credibility of a statement,
   
2. the value or desirability of an action, practice, person, or object,
   
3. the effectiveness and efficiency of a policy, system, design, or object in fulfilling its purpose, or
   
4. the beauty of a creation.

Critical thinking in ‘pure research’ (i.e., curiosity driven research aimed at understanding) is of type A, which requires us to think critically about knowledge claims (statements which are alleged to be true.) Critical thinking in ‘applied research’ (i.e., usefulness driven research aimed at improving ourselves or the world around us) engages in critical thinking of types A-C. Critical thinking in the domain of aesthetics (literature, music, dance, painting, ...) primarily involves type D critical thinking, but it may also involve types A and B, especially in literature. The evaluation of an Honours Thesis calls for A, while the evaluation of the teaching of a faculty member involves A- C, and that of play involves A, B and D.

From the point of a view of the reader of a textbook, journal paper, or PhD thesis, critical thinking of type A boils down to the consideration of the following questions:

1. What is/are the central claim(s) of the author?
   
2. What is the justification that the author provides in support of the claim(s)?
   
3. How sound is the justification?
   
4. On the basis of the author’s justification and other relevant considerations that the author may not have mentioned, how credible is/are the claim(s)?

Justification may be viewed as the demonstration that the claim in question follows as the conclusion from the grounds acceptable to the writer (the one who defends the claim) and the reader (the one who questions the claim). In some cases, the writer can justify a claim by pointing to the grounds as evidence; in other cases, justification calls for a chain of reasoning that connects the grounds to the conclusion. Thus, if you asked me to justify my belief that I have five fingers on my hand, I can simply show you my hand, and say “See for yourself.” Pointing to the relevant evidence is sufficient in this case. However, if you asked me to justify my belief that my great great grandmother (whose photographs I have not seen) had five fingers, I will have to say something like the following:

You and I agree that

1. Most human beings have ten fingers;
   
2. great great grandmothers of human beings are human beings; and
   
3. I am a human being.

From (ii) and (iii), it follows that my great great grandmother was a human being. From this result and (i), it is reasonable to conclude that it is most likely that my great great grandmother had five fingers.

Statements (i)-(iii) constitute the grounds for justifying my belief. Given these mutually agreed upon premises, we can show that claim follows as a rational conclusion from the grounds.

On the basis of the above examples, we may say that a justification consists of grounds and a conclusion based on the grounds, with reasoning to connect the two when necessary. Certain kinds of justification also presuppose a shared value system and criteria. Consider the following arguments:

Gret Slibins is excellent at organizing his materials and communicating his ideas clearly and interestingly, making even the most difficult concepts easy for students to understand.
Therefore he is an excellent teacher.

Bret Glibins is excellent at designing tasks that help students learn on their own and engage in critical thinking and inquiry.
Therefore he is an excellent teacher.

Whether or not we accept the conclusions in these examples depends on our value system with respect to teaching excellence, and the criteria for evaluating teaching excellence derived from these values. If organizing and communicating ideas clearly and interestingly are the primary criteria for teaching excellence, then the conclusion that Gret Slibins is an excellent teacher is justified. But if helping students to acquire higher order thinking abilities is the mark of an excellent teacher, then this conclusion is not justified. In contrast, under this criterion, the conclusion that Bret Glibins is an excellent teacher is justified.
Similar remarks apply to reasoning intended to support conclusions such as the following:

Euthanasia is moral.
Loss of chastity before marriage is immoral.
Matrix is a better movie than An English Patient.

Criteria of evaluation are not restricted to decisions on excellence, morality, etc. They are also relevant to decisions about credibility (truth). The criteria for the evaluation of credibility in all forms of rational inquiry include absence of logical contradictions. In evidence-based inquiry (whether scientific or humanistic), the criteria also include fit with experience. Scientific inquiry includes additional criteria like correctness of predictions, generality, and simplicity. It is only in the context of such criteria that we can judge the soundness of an argument.

Given the above remarks, the structure of justification can be outlined as follows:

Type 1 Type 2 Conclusion Grounds Conclusion      Reasoning Grounds Value system and criteria of evaluation

Given the above model of justification, we may say that the critical evaluation of the soundness of justification of knowledge claims (item (3) under credibility) includes the evaluation of:

• the acceptability of the grounds,
  

• the legitimacy of the reasoning, and
  

• the appropriateness of the value system and criteria of evaluation.

Contributed by K P Mohanan ().