Here is the transcript from my course on Critical thinking in Global Challenges from the University of Edinburgh by Professor Mayank Dutia and Dr. Celine Caquineu.
The most fundamental concept and the starting point for critical thinking is to understand what it is that we are talking about, In other words what do we actually know.
A fact is something which is demonstrably true. It is something that can be checked and verified, and confirmed to be true. For example, it is a fact that the sun rises every morning, in a direction that we call east in English, and it sets every evening in the direction called west. It is a fact that water flows downhill, and that plants grow towards sunlight. The starting point in analysing information is to understand what are the facts, that are relevant to the discussion.
An assertion is a statement that is held to be true, but which is not actually a fact because it is not, or cannot be, proved to be true. For example, you may assert that most people in this building like pizza, but you may not be able to actually prove that unless you actually ask every single person. It is often quite important to understand whether something is a fact, or an assertion. Assertions can be wrong, but facts by definition, are true.
Not too long ago, people believed that the Earth was the centre of the Universe and the sun went around the Earth, and this was accepted as a fact. Actually that was an assertion, which was proved to wrong when we discovered more information about the solar system and the universe that we live in.
Finally an opinion is weaker than an assertion, it is a statement of something that is believed to be true, but it is subjective and it is dependent on the point of view of the person who is making the judgement. As you know, different people may have different opinions on things, at the same time, and people can change their opinions, It is always important to understand when someone is expressing his or her opinion, instead of stating a fact that is demonstrably true.
So these are some of the fundamental building blocks for critical thinking.