I notice that your question hasn't been answered yet, but it has been waiting for one for a while. I feel a little out of my depth here, but just to provoke some further response(s). I'll take a shot. Your question is really about what it means to say that something causes something else. If we think of causation as "deterministic," then the laws of causality work in such a way as to have it be that a given cause will always produce a given effect, and the appearance of this effect can be wholly explained in terms of that cause. There are some scientific fields that seem to work this way--classical mechanics, for example. But then, even in the field of physics, it seems that at some levels this (deterministic) conception of causation seems not to apply--for example, within quantum mechanics. Those who explain what is called the "indeterminacy" at this level might explain it in different ways, but at least in terms of explaining and predicting, it looks like the simplicity of a deterministic model...

It looks to me as if your question is a version of what epistemologists have come to know as "the problem of the criterion"--in this case, with respect to logic. There have come to be three different ways of responding to this problem (which one could also apply to any other sources of information or reasoning, such as sense perception, memory, or induction): reductionism, which provides evidence for the reliability of the source by getting confirmation for that source from some other source (this is more or less the second option you provide in your question, though I am not sure why that seems to you to show that "sometimes logic does not work"); dogmatism, which essentially says that we can be justified in accepting individual samples from a given source without having any (prior) justification for thinking that the source is reliable (this is more or less the first option you present in your question; and holistic coherence theory, which claims that our justification for thinking that the...

Fallacies are forms of reasoning that fail to provide support for the conclusions reached via that reasoning. In other words, the premises could all be true, but the conclusion still false. Just because something is a fallacy does not make the conclusion of such reasoning false, however. For example, here is a (deductive, logical) fallacy with a true conclusion: If my name is Nicholas D. Smith, then I have a very common last name. I do have a very common last name. Hence, my name is Nicholas D. Smith. All the premises are true, and so is the conclusion, but the reasoning is fallacious (called "affirming the consequent"), because the truth of the premises does not in any way support or ensure the truth of the conclusion. To see this, consider another example of the same sort of inference (affirming the consequent): If I am swimming, then I am wet. I am wet. Hence, I am swimming. Nah! I live in Portland, Oregon--folks here are wet most of the year from the rain. ...

Study logic. Then study more logic. And then...study a lot more logic. And then hope that the world (and job market) somehow gives you the opportunity to study and do logic for a living (because if you are distracted with other things, you will never be as good a logician as you could be with logic as your vocation and avocation, which is how muct academics feel about their subjects.

No, they're not the same. A "slippery slope" argument is one that tries to show that attempting to make a determination about one sort of case will "slide" into another case very like it, and then another very like that one, and so on, until we reach a point where we are no longer willing to endorse the original strategy. For example, a familiar version of "slippery slope" reasoning holds that if we grant rights to gays, then the next thing that will happen is that we have to grant rights to pedophiles, or those who enjoy bestiality or incest. The inference we are suppose to make is that wwe should not take the first step down the slope, as there will be no stopping all of the steps that follow "once you go down that path." As this example shows, many "slippery slope" arguments are simply fatuous, and as a matter of fact none are actualluy logically valid. A reductio ad absurdum, however, is a logically valid form of argument. The way this argument form works is in virtue of a certain fact...

From the point of view of deductive logic, your question is very easily answered: a fallacy is an argument form in which the premises may all be true, but the conclusion false. To prove this, one provides what is called a "counterexample," which is simply a substitution instance that has the above characteristics. For example, take a fairly common deductive fallacy, called affirming the consequent: (1) If p, then q. (2) q. Therefore, (3) p. Here is a counterexample: If 2+2=5 (p), then 5>3 (q) 5>3. Therefore, 2+2=5. In inductive logic, however, fallacies may be controversial, because there can be some unclarity about how to calculate or assign probabilities. An example of this kind of problem (to be brief) can be found in the philosophy of religion. For example, some philosophers have claimed that probability arguments can be used to show that it is more likely than not that our world was created by an intelligent designer, because the likelihood of this world coming to...

There is a very good entry on definitions in the Stanford Encyclopedia of Philosophy (on-line), and a pretty good review of the various fallacies of definition in Wikipedia. Happy hunting!

Philosophers have debated this sort of question, but I think the consensus is that the question is not coherent. Being "all-powerful" obviously does not mean being able to do what is logically impossible. Think, instead, of the concept as making God into a being who can do anything that can (in principle) be done. Since lifting a rock of any size can in principle be done, God can always lift the rock. But making a rock so big that it can't in principle be lifted makes no sense at all, hence God's not being able to do that is no indication of not being all powerful, it is just to recognize that the description" a rock so heavy that God can't lift it" is nonsense.

To begin with, I don't accept your example of the political sign. Putting an endorsing sign up in your yard is not an invalid argument--it is simply an expression of your opinion. If someone else is persuaded to vote for a candidate just because you have expressed your opinion in favor of that candidate, then so be it. It won't be the result of a bad or invalid argument--though perhaps their reasoning might be faulted as unsound: If S is going to vote for C, then I should vote for C. S is going to vote for C (I can tell from the sign in S's yard). Hence, I should vote for C. This isn't invalid, but at least the first premise (and perhaps the second, too, should be rejected.) On your main question, however, I do think there is somthing wrong with using invalid arguments (at least ones that we know are invalid) to persuade people, both because it might habituate them into bad reasoning habits more generally, and because it is a kind of seduction. Consider: suppose you really believe...

I think the only way both people could be right is if they don't mean the same thing by "married." Here is a case that might go like that. Suppose the first person is thinking of marriage as a holy sacrament in their religious sect. According to that sect, same-sex marriage is an abomination. Because of that sect's point of view, then, someone might think that there should be no right of same-sex marriage within that sect . Now, even if that is a strongly held belief of that religious sect, it is quite another thing to try to enforce one religious sect's view of things as a matter of law for the rest of the nation (or world). So someone else might think of marriage as a legal contract between two people, one that protects certain civil rights they can enjoy as a result (such as the right to adopt a child as a married couple, for example). It might be that same-sex marriage is an abomination according to some religious groups, but also should be legally permitted as a civil right.