Wilhelm's character was a complete contrast to that of his brother. As a boy, he was strong and healthy, but while growing up he suffered a long and severe illness which left him weak the rest of his life. He had a less comprehensive and energetic mind than his brother, and he had less of the spirit of investigation, preferring to confine himself to some limited and definitely bounded field of work. He utilized everything that bore directly on his own studies and ignored the rest. 