Saturday, July 11, 2009

Soundness and Cogency

When arguments are made for one point of view or another, they are presented with various types of logic: deductive, inductive, and sometimes, abductive. For example, one may argue for naturalistic evolution, theistic evolution, young earth creationism, old earth creationism, or a variety of intermediate positions using logical arguments.

Deductive logic and inductive logic both offer supporting evidence (premises) to justify a claim (conclusion). A valid deductive argument produces a conclusion which is certain. A strong inductive argument produces a strongly probable conclusion. Both types of arguments use logic in their argument forms.

In my July 7 post I stated that Dr. John Ankerberg had received a well-written letter from a young earth listener who used “tight logic.” This statement needs some explanation. Deductive logic, applicable to a rather limited number of discussion topics, virtually guarantees validity. Inductive logic is applicable to a greater number of arguments where high probability of a true conclusion is the best we can hope for. The most important factor in our discussions of evolution or creationism is often overlooked: In order for a deductive argument to be considered sound (reliable), or in order for an inductive argument to be considered cogent (convincing), all premises presented to support the argument must be true or acceptable before one can be assured of a true conclusion.

An argument may be tightly logical, but many arguments in evolution/creationism discussions break down because false or unacceptable premises are used. False premises, if accepted, lead to false conclusions. A deductive argument may be logically valid and an inductive argument may be strong, but if the argument is based on false or unacceptable premises, its conclusion cannot be true.

To illustrate, here are three premises upon which young earth conclusions rest: (1) ALL physical death in the animal world was caused by Adam’s sin in the Garden of Eden 6000 years ago. (2) The Bible should be the ONLY source of knowledge with respect to the history of the earth and life on earth, so we should reject any and all science which points to an old earth. (3) Plants are less than living things, even though it appeared that plants “died” when animals consumed them prior to Adam and Eve’s fall into sin. God did not assign “life” to plants in the same manner he assigned life to animals. Acceptance of the truth of each of these premises supports the conclusion that the earth is young. But if these premises are false or unacceptable, the conclusion of a 6000 year old earth is not demonstrated to be true.

Theologian Kenneth Samples in A World of Difference, states “Understanding that the premises of an argument must be true, relevant, adequate, clear, and knowledgeable helps keep a person’s thinking on the right t-r-a-c-k.” Our Creator has endowed us with the ability to think clearly and soundly. It is our responsibility to use our ability to think clearly and soundly in order to evaluate the beliefs we hold. Proper use of that ability is crucial for our effective witness.