“Alien abductors have asked him to probe them.” “Sasquatch has taken a photograph of him.” The “him” is the “Most Interesting Man in the World,” the faux character in the Dos Equis beer ad campaign, and these are my favorite skeptical lines from a litany of superfluities and braggadocios. (“In a past life, he was himself.”)

My candidate for the most interesting scientist in history I'd like to have a beer with is Alfred Russel Wallace, the 19th-century naturalist and co-discoverer (with Charles Darwin) of natural selection, whose death centennial we will mark this November. As I document in my 2002 biography of him—In Darwin's Shadow (Oxford University Press)—Wallace was a grand synthesizer of biological data into a few core principles that revolutionized biogeography, zoology and evolutionary theory. He spent four years exploring the Amazon rain forest but lost most of his collections when his ship sank on his way home. His discovery of natural selection came during an eight-year expedition to the Malay Archipelago, where during a malaria-induced fever, it struck him that the best fit organisms are more likely to survive and reproduce.

Being open-minded enough to make great discoveries, however, can often lead scientists to make great blunders. Wallace, for example, was also a firm believer in phrenology, spiritualism and psychic phenomena, evidence for which he collected at séances over the objections of his more skeptical colleagues. Among them was Thomas Henry Huxley, who growled, “Better live a crossing-sweeper than die and be made to talk twaddle by a ‘medium’ hired at a guinea a séance.”

Wallace's adventurous spirit led him to become ahead of his time in opposing eugenics and wasteful militarism and in defending women's rights and wildlife preservation. Yet he was on the wrong side when he led an antivaccination campaign. He was a first-class belletrist, but he fell for a scam over a “lost poem” that Edgar Allan Poe allegedly wrote to cover a hotel bill in California. Worst of all, he scientifically departed from Darwin over the evolution of the human brain, which Wallace could not conceive as being the product of natural selection alone (because other primates succeed with much smaller brains) and thus must have been designed by a higher power. Darwin snarled, “I hope you have not murdered too completely your own and my child.”

Wallace is the prototype of what I call a “heretic scientist,” someone whose mind is porous enough to let in both revolutionary and ridiculous ideas at the same time. Other such examples abound in astrophysicist Mario Livio's 2013 book, Brilliant Blunders (Simon & Schuster), in which he skillfully narrates the principle that “not only is the road to triumph paved with blunders, but the bigger the prize, the bigger the potential blunder.” Livio's list includes Darwin's stumble in postulating the incorrect theory of pangenesis, based on the inheritance of particles he called gemmules that carried traits from parents to offspring; Lord Kelvin's gaffe of underestimating the age of the earth by almost 50 times, not because he ignored radioactivity, Livio argues, but because he dismissed the possibility of heat-transport mechanisms such as convection; Linus Pauling's misstep in building a DNA model as a triple helix inside out (because he rushed his research in the race against Francis Crick and James Watson); Fred Hoyle's bungle of siding with the steady state model of the universe over what he dismissively called the “big bang” model despite overwhelming evidence of the latter.

As for Albert Einstein's “biggest blunder” of adding a “cosmological constant” into his equations to account for the expanding universe, Livio claims Einstein never said it: instead Einstein applied the notion of “aesthetic simplicity” in his physical theories, which led him to reject the cosmological constant as an unnecessary complication to the equations.

How can we avoid such errors? Livio quotes Bertrand Russell: “Do not feel absolutely certain of anything.” He then conveys a central principle of skepticism: “While doubt often comes across as a sign of weakness, it is also an effective defense mechanism, and it's an essential operating principle for science.”