Professor Bloggs is an easy grader, and students flock to his courses in droves, because he will give an A to a student just in case the student turns in all the assignments. The easiest answer to your question (other than showing in my example that the logical understanding of "just in case" is one legitimate sense of this term) is that the other senses cannot be reduced to the sorts of truth-functional connectives that are required in logic. But plainly, this is one of those expressions that can be used in several different ways--and thanks to Peter Smith for giving another clear case of this (where "just in case really means "because")!

"Consistent" indicates the logical condition of not being contradictory. In logical contexts, "coherent" is used synonymously with "consistent." But in other contexts, "coherent" is intended to make a stronger connection between two things than the (very weak) connection of being non-contradictory. So, for example, those who claim that "coherence with one's other beliefs" (or some variant of this) is a necessary condition of knowledge do not think that mere consistency is enough. Rather, to say that a believe "coheres" with one's other beliefs is to say that there is some evidenciary or justificatory connection between the coherent items. A very crude, but perhaps useful way to think of this is as follows: Conceive of "consistency" as simply meaning that the items in question do not clash with one another or rule each other out; conceive of coherence as the condition in which the items in question actually "fit" with one another.

When philosophers call something "circular," they generally are making a logical characterization--it means that someone has somehow assumed the truth of what he or she is supposed to be proving, and thus "derives" the conclusion only by having the proposition expressed in the conclusion already present (and underived) within the premises of their argument. An example might help. Suppose you asked me if there was any "proof" that God existed, and I produced the following argument: (1) God says He exists. (2) Because He is God, He would not lie. (3) Hence, God exists. I doubt that you would find this very compelling as a "proof," because both premisses (1) and (2) assume the existence of God, when that is the very thing I was supposed to prove to you. Here's the circle, briefly: How do I know that God exists? Because He said so. How do I know there is a God who could say such a thing? Because He exists. Feel like we're not getting anywhere here? It's because we're not--we're going...

It may be that two distinct properties (in your example, the property of being a spinach-eater and the property of being strong, if your mother were right--which, I fear, she is not!) have the same extensions--that is, may apply to all and only the same things in the world. In this case, the set of all spinach-eaters would be identical to the set of all strong things, since as you say, the identity of sets is determined wholly by membership. But that does not mean that the property of being a spinach-eater is the same as the property of being strong. One reason for thinking that it is not is the two properties would appear to have different causal or explanatory relations--one becomes strong by eating spinach (eating spinach is what explains becoming strong) , but one does not become a spinach-eater by (first) becoming strong (becoming strong is not what explains becoming a spinach-eater). Another famous example from philosophy: assuming there is an omniscient and omnibenevolent God, then it...

I see nothing at all about "common sense" that would make it inherently "unphilosophical." In fact, periodically throughout the history of philosophy there have arisen movements among philosophers in which "common sense" was embraced as providing the very best grounds for philosophical views. I think the only concern any serious philosopher would have about the use of "common sense" within philosophical contexts would be when whatever is counted as "common sense" becomes valorized in a way that puts it beyond dispute. Philosophers, most of all, are required to "think outside the box," as the saying goes. Just because something is widely accepted does not make it immune to philosophical criticism--indeed, the more widely and uncritically accepted something is, the more likely it is to mask some error that will lead us astray. "The unexamined life is not worth living for a human being," as Socrates said (Plato, Apology 38a). That does not make what most people take for granted wrong or...

If no one had asked this question, I wouldn't be answering it! Of course counterfactuals can provide good reasons for thinking certain things. (I'm inclined to add that if they weren't, we wouldn't use them so much...) The very fact that something is recognizably true gives us reason to believe, right? And this is recognizably true: If no one had asked this question, I would be doing something else right now. In fact, I am inclined to think that counterfactual reasoning is so deeply embedded in reasoning itself (especially deliberative reasoning, or choosing between different possible means or ends from which to choose) that we couldn't reason without them. In other words, if it weren't for our ability to use counterfactual reasoning, we couldn't reason at all! OK, I admit it--I'm having some fun here. If I weren't, I wouldn't have answered this question.