The Government sued in 1939 to cancel a certificate of citizenship, granted in 1927, charging that it had been "illegally procured," in that the defendant, at the time of the naturalization and for five years preceding, was not attached to the principles of the Constitution, but was, in fact ,a member of, and affiliated with, and believed in and supported the principles of, certain communistic organizations in the United States which were opposed to the principles of the Constitution and advocated the overthrow of the Government of the United States by force and violence. In construing the Acts of Congress governing naturalization and denaturalization, general expressions should not be so construed as to circumscribe liberty of political thought. Assuming that, in the absence of fraud, a certificate of citizenship can be set aside under ยง15 of the Naturalization Act of 1906 as "illegally procured" because the finding by the naturalization court that the applicant was attached to the principles of the Constitution was erroneous, the burden is upon the Government to prove the error by clear, unequivocal and convincing evidence a mere preponderance of evidence which leaves the issue in doubt will not suffice.