An essential characteristic that qualifies an argument as a deductive argument involves the argument’s starting point. A deductive argument takes a general statement or principle known as correct or factual as a starting point. Deductive arguments then work through the use of a logical structure to prove a point of greater specificity than — and dependent upon — the initial premise of the deductive argument.
Within the structure of a deductive argument, the conclusion cannot be other than true, provided that the initial premise represents a true condition. The reasoning “Massachusetts is a part of New England. Boston is a city in Massachusetts. Therefore, Boston is a city in New England” illustrates this method of taking a known truth of a general nature and devising a deductive argument that makes the conclusion necessarily true because the initial premise is true.
Deductive arguments deal in such matters as association by definition, or as mathematical principles such as the nature of elements in an equation. For example, the well-known mathematical formula that provided that “a” and “c” equal each other when “b” equals “a” or “c,” then “b” must equal both “a” and “c” illustrates a deductively argued mathematical principle. A less commonly thought-out application of this two-way principle applicable to equations involves the premise of the relationship of money and time. You may recognize the truth of the cliché that “time equals money.” You may not recognize, however, that within the two-way principle of mathematical equations this means that the opposite must also hold true. If “time equals money” then, of necessity, “money” must also equal “time.” Content of deductive arguments incorporates such provably valid premises as mathematical principles.
For a deductive argument to have validity, the initial principle must hold true. For example, an attorney make a case that, "The defendant attended an awards dinner at the same time that the crime occurred. The defendant remained within audience view throughout the time of the crime’s occurrence. Therefore, because because the defendant cannot have been present at the crime scene, the defendant could not have committed the crime.” But the alibi’s validity becomes compromised if, for example, the defendant has an identical twin. In such an instance, the belief of eyewitnesses of having seen the defendant at a particular event may not guarantee that the attendee was the defendant versus the identical twin. In this case, the deductive argument has a potentially faulty initial premise, which means that the conclusion also cannot be held as guaranteed.