Among the informal logical fallacies, one of the most common is called the post hoc ergo propter hoc fallacy. Translated it means “after this, therefore because of this.” B follows A, and it is therefore assumed that A must have been the cause of B. This fallacy is so prevalent that it could easily be assumed that many have done graduate studies in it, and some have even moved on to post hoc post doc work.
The problem is complicated because B frequently is caused by A. God placed us in a world where we see cause and effect happening all the time. But for every antecedent real cause there is a multitude of antecedent non-causes. I am about to type the letter t — there! — and this was caused by my left index finger pushing on a key with a t on it. But at the very same moment, there were a bunch of objects on my desk that were not pushing that key, along with numerous objects in the world that were not doing it either. In order to cause something, more is necessary than to simply exist prior to that event.
But sometimes the events that are not “causing it” occur in such a way that it is reasonable to assume that they might have. This is where a lot of post hoc thinking comes in. You eat at a particular restaurant, order the salmon, and two hours later you fall deathly ill. Even if you find out later that it was the flu and had nothing to do with the salmon, it might be years — depending on how sick you got — before you can even look at salmon, or go to that restaurant. In other words, even our bodies have a strong tendency to argue post hoc. Mark Twain once said that a cat that sits on a hot stove plate will never sit on a hot stove plate again — but neither will it sit on a cold one.
All this appears to be a design feature, and quarreling with it is just sweeping water uphill. But God did not just give us the power of association, He also gave us the power of thought. “Maybe it was something I ate” is a reasonable question. Connecting various possible associations is indeed a rational way to form hypotheses. It is an incoherent way to draw conclusions. A reasonable man ought wonder if he is allergic to this or that if he constantly reacts with a rash whenever he eats it. That question ought to lead to some kind of experimentation and thoughtful research. If he wonders if he is lactose intolerant, for another example, then it reasonable to knock off the dairy for a bit and see what happens. Read up on it. Talk to a doctor.
What is not reasonable is to wonder if that is the case, listen to a friend who diagnosed himself with the same condition, note a couple of similarities, read one article on the Internet about it (at TrustworthyAnswers.com) and form a dogmatic opinion that will then be defended to the last ditch. What this approach will result in is a Christian community full of allergies, half of which are genuine and half of which are not, and a host of table fellowship issues.
If you are invited over to someone’s house and you bring your own food because, you say, “I’m a picky eater,” this is what would be called indefensible. So nobody does that. It has to be cloaked in medical garb — “I have allergies” — and that way everybody has to leave you alone.
When the allergies are the real deal, then it would be churlish in the extreme to fault someone for informing the hostess of his allergies beforehand. Better that than to have the guest swell up during dinner because of the nuts he ate, and the whole group has to interrupt the proceedings go down to the ER — taking dessert with them to be finished in the waiting room.
Fuzzy thinking is therefore a sin against charity. There is no way to falsely associate B with A as a consistent pattern of thought without uncharitably disassociating yourself from your neighbor. A man who lived as a hermit on the mountain could falsely assume that his rooster caused the sun to rise by his crowing, and no one is really the worse. But in community, careless post hoc thinking causes real problems. Table fellowship is really important to God, and we ought not allow anything to disrupt it except for a strong reason. People who really do have allergies are not sinning against anyone or anything. But the same cannot always be said for people who mistakenly and carelessly think they do.