Critical thinking is the foundation skill for any profession. Most of us in the engineering professions learn critical thinking skills from our science and math teachers. We also learn them from our professional mentors and experience.

In this time when critical thinking is being challenged by political decision makers it is particularly important to reflect on just what critical thinking is. This list of Carl Sagan’s “suggested tools for skeptical thinking” is a good start. The list has been extracted from his best selling book, The Demon Haunted World. The list is included in Michael Shermer’s Baloney Detection Kit.

The list follows:

1) Whenever possible there must be independent confirmation of the “facts.”

2) Encourage substantive debate on the evidence by knowledgeable proponents of all points of view.

3) Arguments from authority carry little weight — “authorities” have made mistakes in the past. They will do so again in the future. Perhaps a better way to say it is that in science there are no authorities; at most, there are experts.

4) Spin more than one hypothesis. If there’s something to be explained, think of all the different ways in which it could be explained. Then think of tests by which you might systematically disprove each of the alternatives. What survives, the hypothesis that resists disproof in the Darwinian selection among “multiple working hypotheses,” has a much better chance of being the right answer than if you had simply run with the first idea that caught your fancy.

5) Try not to get overly attached to a hypothesis just because it is yours. It’s only a way station in pursuit of knowledge. Ask yourself why you like the idea. Compare it fairly with the alternatives. See if you can find reasons for rejecting it. If you don’t, others will.

6) Quantify. If whatever it is you’re explaining has some measure, some numerical quantity attached to it, you’ll be much better able to discriminate among competing hypotheses. What is vague and qualitative is open to many explanations. Of course there are truths to be sought in the many qualitative issues we are obliged to confront, but finding them is more challenging.

7) If there’s a chain of argument, every link in the chain must work (including the premise) — not just most of them.

8) Occam’s Razor. This convenient rule-of-thumb urges us when faced with two hypothesis that explain the data equally well to choose the simpler.

9) Always ask whether the hypothesis can be, at least in principle, falsified. Propositions that are untestable, unfalsifiable are not worth much. Consider the grand idea that our Universe and everything in it is just an elementary particle — an electron, say — in a much bigger Cosmos. But if we can never acquire information from outside our Universe, is not the idea incapable of disproof? You must be able to check assertions out. Inveterate skeptics must be given the chance to follow your reasoning, to duplicate your experiments and see if they get the same result.

10) The reliance on carefully designed and controlled experiments is key. We will not learn much from mere contemplation. It is tempting to rest content with the first candidate explanation we can think of. One is much better than none. But what happens if we can invent several? How do we decide among them? We don’t. We let experiment do it.

It is one thing to sit alone and think critically about a personal decision like, “Should I change my career?” It is a whole different matter to help a team of executives, stakeholders, and experts “think” critically about a strategic decision like what products should we include in our portfolio or which technology should we invest in? This is the challenge faced by decision engineers. The 10 tools above are essential.