Immanuel Kant articulated his primary position on critical thinking in his essay "What is Enlightenment?" In there he defines enlightenment as "man's emergence from his self-imposed immaturity" and he defines immaturity as "the inability to use one's reason without the guidance from another." What he means by this is that critical thinking is thinking for oneself and so he affirms the Latin term "Sapere Aude!" which means to have the courage to use one's own understanding.
Kant thinks that people are lazy and do not want to use their understanding and do not want to think for themselves. They prefer to let others think for them. So they pay their doctor to give them a diet or prescription. They let their pastor serve as their conscience. They let books do their thinking for them. What their doctor or pastor says is right and cannot be questioned. Books must be right because, after all, they are in print and so their authority is impeccable. We not only believe that we are ignorant in comparison to those who have expertise that we do not have, but we simply are not willing to question them or let what they say come into question using our own reason. We don't want to think for ourselves. We want to be told what to do and what to think.
The remedy for this is not to quit going to a doctor, or to church, and to stop reading, but rather to question the authority of one's doctor and pastor and books. One should seek alternative explanations and directions. One should consider the alternatives and weigh them with what the doctor, pastor or book says. One can always get a second opinion. One should always consider the opposite position. And this is what scholars do.
The private use of reason does not necessarily have to be free. For instance, if you are a pastor you have to follow certain beliefs in order to fulfill your role as a pastor. Otherwise by definition you are not following what is required by the position. A pastor who calls the Apostle's Creed into question is not a Christian. Likewise, it is right to follow the orders in a military context. One should not question the orders and think for oneself when in a situation of war. One has to follow what is prescribed. This is the private use of one's reason.
However, the public use of one's reason needs to be free. This is needed by scholars. The primary symbol for thinking for oneself is the scholar in an academic situation. The scholar needs to be free to question received authority and think through the matter for herself. For instance, it was the received tradition that Moses wrote the Pentateuch of the Old Testament. Scholars who questioned that and began to see reasons why that was not possible suffered ostracism when they challenged that position. it was not until the 20th century that scholars could research and hold a position contrary to that tradition without losing their jobs. But even today, students are often forced to memorize their professors' positions and are not allowed or encouraged to think things through themselves. They are forced to exchange their prejudices for their professors' prejudices instead of being allowed to challenge both opinions. In an academic situation scholars and students should be encouraged to push the envelope and challenge received beliefs so that truth can emerge.
Although Kant is right that the private use of reason restricts thinking for oneself, it is not true that pastors and soldiers and patients don't need to think for themselves. Pastors will meet circumstances and issues that are not dealt with in the bible or the tradition. Soldiers still need to think how best to carry out the orders given to them. Patients may need to trust the expertise of their doctors but they should also seek a second opinion or research the issue on the internet for themselves. So everyone needs to be like a scholar in some sense. We all need to see ourselves in the pursuit of the truth. So good critical thinking is necessary for everyone in every circumstance in life, not just for the scholar.