Hallmarks of Consciousness
By Alexander Mugar Klein
Since the 19th century, philosophers and scientists have failed to agree on reliable markers of sentience.
This Article From Issue
July-August 2026
Volume 114, Number 4
Page 240
A frog will wipe away a spot of acid spattered on its side—even after its entire head has been surgically removed. This strange fact was no surprise in 1853 when a young German physiologist named Eduard Pflüger published his findings; researchers had long known that many reflexes are preserved in brainless vertebrates. But Pflüger’s experiment became a blockbuster because of what happened next.
QUICK TAKE
- In the 1850s, a physiologist discovered that a decapitated frog can try different means to achieve a goal. The study ignited a debate over the true markers of consciousness.
- Researchers consider behavioral markers to determine whether a creature or species is conscious. But they still dispute which behaviors, if any, are reliable indicators of consciousness.
- To move the debate forward, researchers should perhaps heed the advice of the 19th-century psychologist William James and drop the reliance on markers altogether.
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