An argument is considered to be deductive when the

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An argument is considered to be deductive when the premisses present absolutely conclusive  evidence for the conclusion. Thus, a deductive argument can either be valid or invalid. NOTE: An argument is deductively valid if and only if its underlying structure or form guarantees that: if its  premises are all true, then its conclusion is also true. This means that in deductively valid argument, when its premises are true then, it is IMPOSSIBLE  that its conclusion is false. Otherwise, we involve ourselves in a contradiction.
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But an argument is deductively invalid if and only if its underlying structure does not guarantee that;  if its premises are all true, then so is its conclusion. Notice that the above definitions of deductive validity or invalidity are not about the truth of the  premises but rather the structure of the arguments. It is precisely the structure of the argument that  guarantees that provided its premises are all true, then its conclusion will not be false. Let us take an example to illustrate our point: All Senegalese are Africans. President Moi is a Senegalese Therefore, President Moi is an African. We observe that the second premise in the argument above, is false because President Moi is not a  Senegalese. Nevertheless, we are claiming that if these premises were all true, then the conclusion  would also be true. Therefore, the above argument is a deductively valid argument. In pure logic, the truth is not the major concern. Validity instead is the major concern. However, as  critical and creative thinkers we are constantly concerned about whether an argument has true  premisses. Whenever an argument has all true premisses and deductive validity, we say that it is a sound  argument. Otherwise it is unsound. NOTE: An argument is SOUND if and only if it is both deductively valid and has all true premisses.
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The reason why it is impossible for the conclusion of a deductively valid argument to be false if the  premisses are true is that the conclusion makes no factual claim that is not implicitly made by the  premisses. What the conclusion does is merely to lay bare (or explicit) what was initially hidden (or  implicit) in the premisses. So, the conclusion actually does not venture to say anything more than  what the premisses contain. It is noticeable therefore that deductive logic regards argument correctness as all or nothing, i.e.  either it valid or it is invalid. Inductive logic on the other hand, is concerned with connections from  premisses to concussions that allow for graduations. We can hence have a more or less strong  inductive argument.
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