We see a lot of arguments here that involve one poster complaining that the other is using a "fallacy" or flawed argument that does not meet the standards of reasoning in formal logic. I don't think that means such arguments are worthless, only that they do not, by themselves, definitively PROVE that the assertion made is certainly true. We laypeople talking among each other are not debating according to the rules of formal logic. We do not use formal logic in courts of law. Plenty of powerful evidence used in trials and hearings could be challenged as a "fallacy" if used in a high school logic exercise. Appeal to authority? We call these people "expert witnesses." The testimony of such a witness is worth whatever the finder of fact says it's worth. If one expert on vehicle crash dynamics has a Ph.D. in physics and an M.D. too, that person's opinion on what caused the driver's death is more weighty than a beat cop who attended a 20-hour course on investigating traffic accidents. Probability fallacy? We regularly take testimony from people about how things normally operate in their lives, at their business, or in relations with some other person. Tesimony about a regular habit or routine IS admissible evidence that tends to show that, in a particular incident, the person acted consistent with that habit or routine. Ad hominem? Well, this can properly go to the credibility of the person offering the argument. Is she smart, or a fool? Is he biased, or is he neutral or disinterested in the finding or not finding of that fact or condition? Is the person sane? Does the person have good judgment? If a person who is known to use street drugs tells me something, I am less likely to believe her. Such drugs negatively affect both perception and memory. Has the person lied before? As a matter of logic, even a person who has lied 99 times in a row COULD be telling the truth now, answering the 100th question, but it's far more likely that the person is acting in conformity with his habit of lying. I read a Torts (products liability) case where an "expert" testified that the door latch for a certain make and model of minivan was defective. He could tell from the design that it was too easy to pop open in a collision, and laboratory tests proved he was right. On cross-examination, he conceded that no automotive door lock ever used on any vehicle was strong enough to suit him. He considered all of them defective. He had designed a lock mechanism that was good enough, in his expert opinion, but no car company had shown any interest in using it. His credibility was shot. He was a hypocrite. He "said" the door locks on every car that's ever been on the road were defective, but he drove cars with such locks himself, and trusted the lives of his family to such "defective" door latches. Appeal to the masses? The "popular opinion" fallacy? Well, for generations, the law of expert testimony (Frye standard) said that an expert's opinion would always be admissible if it were based on data or methods that were generally accepted in the scientific community. Now, the Daubert standard looks more to the substance of the science, the data, or the experiments, BUT a factor that is STILL worthy of consideration is whether that method is generally accepted as valid in the scientific community. That's bolstering your argument by citing its popularity. A logical fallacy, but not a barrier to legal evidence used to persuade fact-finders in court.