Logic is often attributed with more function that it has ever claimed. I hear, from time to time, an interlocutor claim that “Logic tells us that X is right” or “Logically, such and such is true.” The worst case of this that I experienced was when I was told, “So you’re studying theology. I’m very interested in this… I mean, logic tells us that God doesn’t exist.”
Although the primary reactions are a combination of debate and so on and so on, it was clear that by invoking logic he had attempted to take the rational high ground. In other words, by being the first person to claim that his position is the result of logic, any contrary position could not be founded on logic, and if not based on logic it could not be true. Now, in the current state of affairs, rationalism is given the highest standing as the most reliable basis on which to assert a truth. It is, however, a trick of persuasion and debate rather than an actual exercise in logic.
Logic, after all, is only really used to test propositions or to reach conclusions from propositions. For example, “The sky is dark for about half the day because the sun is located on the other side of the planet” is a proposition that logic can test. Also, “The sky is dark for about half the day because humans generally prefer to sleep at night” is another. Likewise, logic can be used to extrapolate from, “This house is made of wood, and wood is combustible” to “Fire can destroy this house” or even “If I want to protect this house I should ensure that it does not catch fire.”
The point is that without an initial proposition, logic doesn’t have a starting position and cannot then reach any conclusions. Logic, by itself, can tell us nothing.
However, what the invocation of logic does tell us is the fundamental proposition or assumption of the invoker. In the creationist argument, one person might say that, “Just look at creation! That’s enough argument to tell me that God exists.” The unspoken assumption is that only a God could create such a thing, that there is no way the universe could spontaneously come into existence. Alternatively, one might weigh in to a debate on abortion and say that, “Early term abortion is acceptable because the foetus has not yet developed a neural system.” Again, the unspoken assumption is probably that the validity of human life is based largely on the presence of a healthy neural system that can support consciousness, rather than the assumption that human life is made valid by the presence of a soul or spirit that inhabits the flesh.
Logic, by itself, tells us nothing about the claim. When invoked by a lazy debater, however, it tells us that not only is the debater lazy (or a practitioner of misdirection) but it reveals the underlying assumption on which the conclusion is based. And for that there is only one remedy, the assumption must be exposed for what it is, an unfounded assumption. It is merely an assertion, based only on the force of will of the one making the claim. An assertion is only that, an assertion of the will, and it holds weight only by conviction or violence. Logic, so very often, doesn’t tell us what we claim it does, but it reveals so very much about ourselves.