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What’s the temperature outside? At the moment I’m writing this it’s 82º. That’s an opinion. It’s not an opinion for being mine; it’s an opinion because I’m giving no reasons for believing it’s true. We have my assertion that it’s a certain temperature outside with nothing other than my personal authority, if that, to support it. It’s open to some criticism. We can ask if it’s plausible. We can get a hint of its accuracy by the exactness of the number. But even if I’m an authority giving you my expert opinion, it’s a bare assertion of what I believe.

How did I “know” it was 82º outside? Perhaps I saw it on TV, or on a sign while I was driving. Maybe I checked my smartphone. Each of these things is an argument, a reason, or reasons, for believing that’s the temperature.

ARGUMENT = REASON(S) FOR BELIEVING SOMETHING IS TRUE

Now you can make a more detailed assessment as to just how likely my claim is. You also have a means for answering this kind of question yourself. You're actually learning how to think. Ralph Nader (starting at 53:30 in this video) said his father used to ask him: “What did you learn in school today. Did you learn what to believe or how to think?” Much of education today is about opinions, what to believe; this book is about arguments, how to think.

All we ever have are arguments for what we think we know is true.  Some will insist there’s some kind of direct access, faith perhaps or what we’ve been taught.   The culture of any group often comes down to what is a set of shared opinions.  And, it’s not unusual for them to insist those beliefs are true, that it’s their opinion, and that’s the end of it.  This is a ploy — a fallacy known as “fanatical closure”The refusal to discuss any arguments for or against a particular claim. — to cut off any discussion. Some feel Eve’s original sin was her willingness to even discuss God’s prohibition of eating from the tree of knowledge with the serpent.  Yet there are arguments, there are reasons, for these opinions, even if they are implicit and amount to no more than some authority.  We only have arguments, and there’s always is an argument, for anything and everything we think is true.