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Critical Thinking for Transformative Justice

Critical Thinking for Transformative Justice

Deductive reasoning,

Deductive reasoning, also deductive logic or logical deduction or, informally, "topdown" logic, is the process of reasoning from one or more statements (premises) to reach a logically certain conclusion. Deductive Reasoning then states that "Socrates" must be "mortal" because he inherits this attribute from his classification as a "man". Deductive reasoning links premises with conclusions. If all premises are true, the terms are clear, and the rules of deductive logic are followed, then the conclusion reached is necessarily true. Deductive reasoning (top-down logic) contrasts with inductive reasoning (bottomup logic) in the following way: In deductive reasoning, a conclusion is reached reductively by applying general rules that hold over the entirety of a closed domain of discourse, narrowing the range under consideration until only the conclusion is left. In inductive reasoning, the conclusion is reached by generalizing or extrapolating from initial information. As a result, induction can be used even in an open domain, one where there is epistemic uncertainty. Note, however, that the inductive reasoning mentioned here is not the same as induction used in mathematical proofs – mathematical induction is actually a form of deductive reasoning. An example of a deductive argument: 1. All men are mortal. 2. Socrates is a man. 3. Therefore, Socrates is mortal. The first premise states that all objects classified as "men" have the attribute "mortal". The second premise states that "Socrates" is classified as a "man" – a member of the set "men". The conclusion The law of detachment (also known as affirming the antecedent and Modus ponens) is the first form of deductive reasoning. A single conditional statement is made, and a hypothesis (P) is stated. The conclusion (Q) is then deduced from the statement and the hypothesis. The most basic form is listed below: 1. P → Q (conditional statement) 2. P (hypothesis stated) 3. Q (conclusion deduced) In deductive reasoning, we can conclude Q from P by using the law of detachment. However, if the conclusion (Q) is given instead of the hypothesis (P) then there is no definitive conclusion. The following is an example of an argument using the law of detachment in the form of an if-then statement: 1. If an angle satisfies 90° < A < 180°, then A is an obtuse angle. 2. A = 120°. 3. A is an obtuse angle. Since the measurement of angle A is greater than 90° and less than 180°, we can deduce that A is an obtuse angle. The law of Syllogism takes two conditional statements and forms a conclusion by combining the hypothesis of Page 15 of 45

one statement with the conclusion of another. Here is the general form: 1. P → Q 2. Q → R 3. Therefore, P → R. The following is an example: 1. If Larry is sick, then he will be absent. 2. If Larry is absent, then he will miss his classwork. 3. Therefore, if Larry is sick, then he will miss his classwork. We deduced the final statement by combining the hypothesis of the first statement with the conclusion of the second statement. We also allow that this could be a false statement. This is an example of the Transitive Property in mathematics. The Transitive Property is sometimes phrased in this form: 1. A = B. 2. B = C. 3. Therefore A = C. The law of Contrapositive states that, in a conditional, if the conclusion is false, then the hypothesis must be false also. The general form is the following: 1. P → Q. 2. ~Q. 3. Therefore we can conclude ~P. The following are examples: 1. If it is raining, then there are clouds in the sky. 2. There are no clouds in the sky. 3. Thus, it is not raining. Deductive arguments are evaluated in terms of their Validity and Soundness. An argument is valid if it is impossible for its premises to be true while its conclusion is false. In other words, the conclusion must be true if the premises are true. An argument can be valid even though the premises are false. An argument is sound if it is valid and the premises are true. It is possible to have a deductive argument that is logically valid but is not sound. Fallacious arguments often take that form. The following is an example of an argument that is valid, but not sound: 1. Everyone who eats carrots is a quarterback. 2. John eats carrots. 3. Therefore, John is a quarterback. The example's first premise is false – there are people who eat carrots and are not quarterbacks – but the conclusion must be true, so long as the premises are true (i.e. it is impossible for the premises to be true and the conclusion false). Therefore the argument is valid, but not sound. Generalizations are often used to make invalid arguments, such as "everyone who eats carrots is a quarterback." Not everyone who eats carrots is a quarterback, thus proving the flaw of such arguments. In this example, the first statement uses categorical reasoning, saying that all carroteaters are definitely quarterbacks. This theory of deductive reasoning – also known as term logic – was developed by Aristotle, but was superseded by propositional (sentential) logic and predicate logic. Page 16 of 45

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