Like any human activity, the practice of critical thinking requires several assumptions to make sense. For people who don’t share the assumptions, the whole process can be experienced as confusing or nonsensical. Here is a partial list of assumptions that sometimes cause trouble for people new to critical thinking.

1. Critical thinking (CT) is evaluative.  An evaluation is a statement that compares what is the case to a standard about how things should be. CT requires people to make lots of judgments about good and bad, right and wrong, what we should or shouldn’t do. The standard of evaluation used in critical thinking for reasoning is reliability. Good reasoning is reliable, and bad reasoning is unreliable.
2. In CT, reasoning implies evaluation, both individual (“You should recycle your aluminum!”) and collective (“We should abolish the death penalty!”). Each statement can be supported by reasons, and the reasons can be evaluated as better or worse.
3. In CT, truth is treated as absolute — not partial, changing, or relative to different points of view.
4. The ultimate “should” in critical thinking is this: you should not contradict yourself. There are other “should” statements, but they are all based on this idea that self-contradiction is bad. Contradictory statements, by definition, cannot all be true, and based on #3 above that means they can’t be partly true, or true to some people but not others. A statement that is self-contradictory is absolutely, eternally, necessarily and inevitably not true.

If you don’t agree with one or more of the above assumptions, expect some trouble even understanding what is going on when trying to use critical thinking.  The assumptions listed above are offered not to convince you to accept any of these assumptions or to “prove” them in the formal (CT) sense, but just to offer a bit more about what is assumed in this field.

There is also one more assumption to consider, that is people shouldn’t judge other people’s opinions.

Critical thinking requires energetically judging other people’s opinions (along with our own!) – not in isolation, but in relation to each other. That is, CT requires asking if the reason given to support or back up an opinion is a good one. If no good reason can be found to support an opinion, that opinion is treated as unsupported or unproved. Generally, opinions are better if proved, and not as good if unproved.  By extension, there is a preference for reflective opinions arrived at through slow thinking over opinions of the moment which are formed in fast thinking.

Many people put the majority of their critical thinking energy into judging the thinking of those they disagree with. Our hope is that you will have come to understand that thinking carefully about your own beliefs is worth more of your time, and that you will have come to appreciate the vital importance of people who do not share your same ideas to your process of slow thinking.