Deductive argument consists of the application of deductive processes of reasoning to argumentative discourse. This process of applying logical principles is somewhat more complicated than that involved in induction. In some respects it is more important that the student thoroughly master deduction than it is that he master induction. Fallacies are more easily concealed in the deductive process than in the inductive process. Nevertheless, when the fallacy is once detected it can be set forth clearly by anyone who understands this form of reasoning. Neither the inductive nor the deductive form of reasoning is often found alone. Most arguments contain both of these processes and in some cases they are very closely interwoven. This fact necessitates a thorough study of both processes. From this standpoint a knowledge of one form is as important as a knowledge of the other. In order that we may thoroughly understand the application of the deductive process to argument we must first consider separately that process of reasoning.
I. Deductive reasoning.
By deductive reasoning we arrive at a conclusion regarding a particular person, event, or thing by reason of our knowledge regarding the whole class to which the particular person, event, or thing belongs. In this sense it is the opposite of induction. We conclude that a particular book is interesting because we know that all the books written by the author of this book are interesting. We may say that deductive reasoning begins where inductive reasoning leaves off. For example, we found that we could arrive at the imperfect inductive conclusion that all of Stevenson’s books are interesting because each one of a number of his books which we had read was interesting. Since (1) the number of specific instances cited were sufficient to offset the probability of coincidence, (2) the class was fairly homogeneous, (3) the examples were fair, (4) we found upon investigation that there were no exceptions, and (5) from the character of the author and other circumstances the conclusion seemed reasonable, we concluded that our induction was sound. Now, taking this conclusion as true we may apply it to any one of Stevenson’s works not yet examined and thus determine that that work is interesting. It must be kept in mind, however, that a deduction based upon an imperfect induction is no stronger than that imperfect induction. The imperfect induction gains no strength by reason of its having a valid deduction based upon it. Nevertheless, unsound arguments are often given a superficial appearance of validity by this means.
We may more clearly indicate the relation of the inductive and the deductive process by arranging the material of the foregoing illustration in the following manner.
A. Inductive process.
1. Specific instances.
(1) Treasure Island, written by Stevenson is interesting.
(2) Kidnapped, written by Stevenson is interesting.
(3) David Balfour, written by Stevenson is interesting.
(4) Prince Otto, written by Stevenson is interesting.