From an interview in The Paris Review HERE.
Would you tell us about the motto tacked over your desk?
It says, “Look at your fish.” It’s the test that Louis Agassiz, the nineteenth-century Harvard naturalist, gave every new student. He would take an odorous old fish out of a jar, set it in a tin pan in front of the student and say, Look at your fish. Then Agassiz would leave. When he came back, he would ask the student what he’d seen. Not very much, they would most often say, and Agassiz would say it again: Look at your fish. This could go on for days. The student would be encouraged to draw the fish but could use no tools for the examination, just hands and eyes. Samuel Scudder, who later became a famous entomologist and expert on grasshoppers, left us the best account of the “ordeal with the fish.” After several days, he still could not see whatever it was Agassiz wanted him to see. But, he said, I see how little I saw before. Then Scudder had a brainstorm and he announced it to Agassiz the next morning: Paired organs, the same on both sides. Of course! Of course! Agassiz said, very pleased. So Scudder naturally asked what he should do next, and Agassiz said, Look at your fish.