In the early 1900s Külpe performed experiments on the concept of abstraction at the Würzburg School. Külpe defined abstraction as a process in which one focuses on certain aspects of reality while ignoring others. In one famous experiment Külpe instructed participants to observe a display of numbers, letters, colors, and shapes. For example, if he told the participants beforehand to report on the numbers observed, then they were unable to describe the letters, colors, or shapes with any accuracy after the experiment. If he told participants to describe the colors, then with subsequent questioning they were unable to describe the letters, numbers, or shapes. The item people could describe with the highest level of accuracy was always the item they were instructed to observe. These results indicated that they subjects of the experiment would abstract the requested features, while at the same time remaining “unconscious” to all of the other present features. Külpe wrote in a letter about the conclusions from these experiments: