Recently, several threads have had some debate about logic, evidence, critical thinking and skepticism. I wanted to write a post discussing those things, hopefully to clarify what those things are, why they are important and how to use those tools.
First, logic, at its core, is simply a method for how to reason validly, how to draw conclusions based on a premise. A simple mathematical example is "if A=B then B=A". In computing, logical operators are used often, AND, NAND, OR, XOR, etc., are used to evaluate expressions and return results, for example, if A>0 AND B>0, then return a value of 1, else return 0". If philosophical logic, an example would be "All trout are fish. All fish live in the water. Therefore all trout live in the water". Logic is simply a method for drawing conclusions from facts, a set of premises and other data.
Evidence is that which we use to for conclusions about things. It can take many forms, physical, mathematical, etc.. Determining the quality of evidence is a part of critical thinking.
Critical thinking is a method for analyzing information using logic, experience, deduction, induction, the weighting of evidence, the quality of evidence, etc.
Skepticism is about determining how and when to have reason to believe something to be true or false, whether or not a thing happened or is true or will happen. For instance, I've no problem with believing someone when they tell me they had a sandwich for lunch today. I know sandwiches exists and are common. The evidence I need for that is next to zero. However, if that person had lunch with me and I watched her eat a salad, I've direct evidence to the contrary and have reason to disbelieve them. Were someone to tell me they have an invisible dragon in their garage, the weight of the claim requires more evidence than for a claim of having a sandwich and I've no reason to believe it absent evidence.
All of these are components of quality thinking. One other component is knowledge. For instance, if we were discussing why the Hubble telescope can't see the moon rover or flag left behind on the moon, some knowledge of telescopes and how they work is required to understand it. If you don't have that knowledge and aren't willing to learn about how telescopes work, then your opinion on that doesn't carry any weight. If you haven't bothered to learn about evolution, biology, etc., then your opinion on the truth of evolution doesn't carry any weight.
However, using logic and critical thinking, we can determine whether or not claims make sense. For instance, were someone to make the claim "All fish swim. Penguins swim. Therefore, penguins are fish.", I can logically pointing out, without specific knowledge of which fish, penguins, location, etc., that the statement is logically invalid and the conclusion does not follow from the premise. Similarly, "All fish swim, trout swim, therefore trout are fish" reaches the correct conclusion but from the incorrect premise and logic. It's often called "right for the wrong reasons" or "right by accident". The conclusion, although correct, is logically invalid.
Critical thinking is putting all of this together to come to conclusions. It's using experience, knowledge, logic and evidence to come to a decision. It's realizing when work is needed to gain education to properly understand a subject. It's using experience to know when something makes sense (for instance, if someone were to tell me that my friend who weighs 120 lbs ate 50 lbs of steak in an hour, I don't need extensive biology knowledge, just experience, to know that isn't true) or doesn't.
This is not meant to cover all forms of thinking or to be an exhaustive guide on how to think, but to provide some basics on quality thinking.