The phrase “Absence of evidence is not evidence of absence” is often attributed to Carl Sagan, an American astronomer, astrophysicist, and science communicator. The statement encapsulates an important concept in scientific reasoning and critical thinking: just because there is no evidence for a proposition doesn’t necessarily mean that the proposition is false.
In the realm of science and skepticism, the phrase serves as a cautionary guideline against making definitive claims based on an absence of proof. This principle is often invoked in debates around phenomena that are hard to measure or observe. For instance, in the search for extraterrestrial life, the absence of evidence (no direct contact or signs) does not confirm that life does not exist elsewhere in the universe. In medicine, it warns clinicians not to rule out diagnoses solely based on the lack of definitive tests or visible symptoms.
Carl Sagan, being deeply involved in the popularization of science and skeptical thinking, often emphasized the importance of rigorous methodologies that avoid logical pitfalls like this one. The phrase also finds its resonance in legal settings, where an absence of evidence may not necessarily absolve a suspect of guilt, especially when circumstantial evidence or motive can be established.
Thus, the statement is a plea for epistemological humility. It argues that our scope of understanding is limited by our observational tools, methodologies, and preconceptions. Therefore, absence of evidence should not be misinterpreted as a proof of non-existence, but as an impetus for further inquiry and open-mindedness.
The Hidden City of Troy: A Testament to Absence and Presence
For centuries, the city of Troy was considered a mere myth, a figment of Homeric imagination described in the “Iliad.” No concrete evidence supported its existence, leading many scholars to assert that the city was fictional. In this context, the absence of evidence became a sort of evidence for its absence in the annals of history.
Enter Heinrich Schliemann, a German businessman and a self-taught archaeologist, who was not willing to take the absence of evidence as evidence of absence. Fueled by a passionate belief in the historical accuracy of Homer’s epics, Schliemann began excavations in Hisarlik, Turkey, in 1871. His excavations were met with skepticism and even ridicule. However, Schliemann’s conviction didn’t waver, and he continued to dig.
In 1873, Schliemann hit archaeological gold. He uncovered a city layer replete with artifacts, fortifications, and evidence of destruction by fire, which closely matched the description of Troy in the “Iliad.” Subsequent excavations and studies would further validate the site as the likely historical Troy, turning Schliemann into a hero of archaeology. While some of his methods and claims were later scrutinized, his discovery was monumental.
Schliemann’s discovery of Troy serves as a timeless example of how the absence of evidence is not evidence of absence. For centuries, the absence of Troy in the archaeological record was not due to its non-existence, but due to the limitations in human knowledge and exploration. The story serves as a powerful reminder to maintain epistemological humility and to keep searching, especially when evidence is lacking. Sometimes the truth is just waiting to be unearthed.
The Invisible Epidemic: The Rise of Helicobacter pylori
For much of the 20th century, ulcers were believed to be caused by stress, spicy foods, or excessive acid production in the stomach. Doctors often prescribed antacids, bed rest, and a bland diet as the standard treatment, yet ulcers frequently recurred. The absence of evidence pointing to an alternative cause led the medical community to generally accept this line of thinking.
Barry Marshall, an Australian physician, and Robin Warren, a pathologist, weren’t convinced. They observed spiral-shaped bacteria in stomach biopsies, a discovery that contrad’t fit with existing beliefs. According to established medical knowledge, no bacteria could survive in the acidic environment of the stomach. Because no one was looking for microbial culprits in such an environment, their absence from medical literature was taken as evidence that they did not exist in the stomach.
In an extraordinary move to prove his point, Barry Marshall drank a culture of the bacteria, known as Helicobacter pylori, and developed gastritis, a precursor to ulcers. His self-experimentation and subsequent antibiotic treatment, which cured him, finally provided the evidence needed to show that H. pylori were the primary cause of many ulcers. This revelation revolutionized ulcer treatment, turning a chronic condition into a curable one.
Marshall and Warren’s work, which earned them the Nobel Prize in Physiology or Medicine in 2005, was a stark reminder that the absence of evidence is not evidence of absence. Had the medical community remained content with the prevailing wisdom and not questioned the lack of evidence for other causes, the root cause of ulcers would likely have remained a mystery, and millions would have continued to suffer. Their story exemplifies the critical importance of challenging prevailing narratives when “absence” seems to be the only supporting evidence.