Before examining some formal rules of deductive logic, we should observe that Aristotle invented logic and we should examine the three so-called Fundamental Rules of Thought which he formulated. You may read more about Aristotle’s involvement with the beginnings of logic. https://oregonstate.edu/instruct/phl201/modules/Philosophers/Aristotle/aristotle_laws_of_thought.html
The Three Fundamental Rules of Thought
- The Law of Identity
- Whatever is, is;
- Every A is A.
- The Law of Contradiction
- Nothing can both be and not be
- Nothing can be A and not A.
- The Law of Excluded Middle
- Everything must either be or not be
- Everything is either A or not A.
My variation these laws as state in Stock, St. George William Joseph. Deductive Logic (pp. 16-17). Kindle Edition.
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