Learning to think critically is a habit you develop over a lifetime. Critical thinking is thinking that does not contradict itself and relies, when possible, on evidence.
One reason critical thinking takes a lifetime to develop is because the critical thinker is always rooting out unwarranted assumptions, and incorrect speculations, in her or his own thought.
One of my favorite introductions to the concept of critical thinking is the 'weasel word.'
'Weasel words' are a rhetorical device meant to catch the uncritical thinker unawares. It is a common persuasive device, but relatively ineffectual once you know it exists.
Wikipedia includes a nice list of examples in its entry on weasel words. The bottom line here is to learn to be specific and to point to the research/evidence that backs up your argument. When someone is trying to persuade you while using weasel words ask them to elaborate, to be more specific, or to show you exactly where they got their information.
- "A growing body of evidence..." (Where is the raw data for your review?)
- "People say..." (Which people? How do they know?)
- "It has been claimed that..." (By whom, where, when?)
- "Critics claim..." (Which critics?)
- "Clearly..." (As if the premise is undeniably true)
- "It stands to reason that..." (Again, as if the premise is undeniably true—see "Clearly" above)
- "Questions have been raised..." (Implies a fatal flaw has been discovered)
- "I heard that..." (Who told you? Is the source reliable?)
- "There is evidence that..." (What evidence? Is the source reliable?)
- "Experience shows that..." (Whose experience? What was the experience? How does it demonstrate this?)
- "It has been mentioned that..." (Who are these mentioners? Can they be trusted?)
- "Popular wisdom has it that..." (Is popular wisdom a test of truth?)
- "Commonsense has it/insists that..." (The common sense of whom? Who says so? See "Popular wisdom" above, and "It is known that" below)
- "It is known that..." (By whom and by what method is it known?)
- "Officially known as..." (By whom, where, when—who says so?)
- "It turns out that..." (How does it turn out?¹)
- "It was noted that..." (By whom, why, when?)
- "Nobody else's product is better than ours." (What is the evidence of this?)
- "Studies show..." (what studies?)
- "A recent study at a leading university..." (How recent is your study? At what university?)
- "(The phenomenon) came to be seen as..." (by whom?)
- "Some argue..." (who?)
- "Up to sixty percent..." (so, 59%? 50%? 10%?)
- "More than seventy percent..." (How many more? 70.01%? 80%? 90%?)
- "The vast majority..." (All, more than half—how many?)
¹It is important that real examples do not in fact explain, at a later stage of the argument, what exactly is meant by "it turns out that"; the whole needs to be looked at before it can be decided that it is a weasel term.