At his own criminal trial, Cheek represented himself. He also testified that around 1978 he had begun attending seminars conducted by a group that believed that the Federal income tax system was unconstitutional. Cheek stated that based on the seminars and his own study, he sincerely believed that the tax laws were being unconstitutionally enforced, and that his actions were lawful. Cheek specifically testified about his own interpretations of the U.S. Constitution, court opinions, common law and other materials. He testified that he had relied on those materials in concluding that he was not required to file tax returns, that he was not required to pay income taxes, and that he could claim refunds of the money withheld from his pay. Cheek also contended that his wages from a private employer (American Airlines) did not constitute income under the internal revenue laws. Cheek argued that he therefore had acted without the "willfulness" that was required for a criminal tax conviction.