Hermann Ebbinghaus (1850-1909) is considered a pioneer of cognitive-psychological research. Ebbinghaus was the founder of the experimental study of memory and the discoverer of the learning curve and the forgetting curve. He was also the inventor of the three psychological measurement methods of memory performance that are still valid today: the recognition method, the reproduction method, and the savings method. His experimental approach was novel in that it operated with the learning of nonsense syllables to minimize the errors arising from experiences and content. He was the first to use KVK trigrams to conduct memory experiments independently of the vocabulary of the subjects. In the most well-known version of this illusion, two circles of identical size are arranged close to each other, with one surrounded by larger circles and the other surrounded by smaller circles.