If there was a list of terms that philosophy particularly likes to work on, the word “concept” itself would probably come out on top. This is reflected in the fact that philosophy can be understood as the science that deals with the foundations of all sciences, including their own, and with the possibility of forming the concept in general. So you necessarily have to talk a lot about concepts.
Also at the top of the charts there should be something less obvious: the inconsistency. Practicing philosophy means constantly talking about whether and why something contradicts itself. As a general rule, first, people are supposed to think reasonably and reasonable thinking presupposes that one does not hold contradictory beliefs. For example, it is not reasonable to think at the same time that you should always get up early, but also that you should sleep long. Second, it is often assumed that nothing paradoxical can be true at the same time, regardless of human: my cats cannot be white and black at the same time, regardless of my beliefs about their color.
If one assumes now that there can be no contradictions or that they are all only apparent and are in fact caused by misunderstanding, errors or unclean thinking, then whatever leads to contradictions is necessarily wrong. Starting with the assumption that everything is either true or false, one can lead indirect proofs in this way: if the assumption of the opposite of the sentence inevitably leads to a contradiction, then the sentence must be true. One speaks here of the so-called proof of contradiction or ‘reduction to absurdity’.
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